Hey All!
I need help. I'm a veteran DM, been running since before 3rd Edition even came out. But one thing I've never done is run an ocean game. My current campaign is set around the Mediterranean Sea. So, sailing is not just an option, but practically an expectation.
Ahh... yes. I've got some experience in this field. Ok, what do you need to know?
Those who have been in games aboard ship, how do you make them compelling? Sure, in port it's fun, but the time on ship... How do I make that part fun? I don't want to just gloss it over. I want to make it memorable.
It's fundamentally the same as making travel on land memorable. The first time or two out, you can lavish detail on the travel on a day to day basis, playing out weather, minor encounters at sea, and RP with the crew. So, generate weather, generate random and planned encounters both minor and major, and make sure you have detailed memorable NPC's aboard the ship. After you've established tone and setting, you probably will want to back off a bit because the mundane reutine of travel is eventually going to bore. 'Are we there yet' is a legitimate complaint. Sometimes 'travel by map' is the prefered method.
Minor encounters at sea are ways to establish what life is like at sea. This could be simply seeing merchant vessels from other nations passing by - who exchange knowlege about monsters, weather, political events at ports of call - with your vessel via flag. It can be seeing a fishing fleet of small vessels plying their trade. It can be being boarded by the customs authority of the nation you are planning to visit, looking for contraband or undeclared goods. It could be seeing a plague ship flying a flag that indicates pestilence aboard. It could be seeing prison ships or galleys. It could be having sea gypsies (second class citizens, possibly non-human, who live their whole lives at sea) approach your boat in small vessels hoping to trade goods. It could be seeing the warships of various nations, displaying the flag, escorting convoys, chasing pirates, or manuevering to engage the warships of rival nations. It could be encountering creatures that live at sea that are presently non-hostile. For example, in my world, there is a type of small drake that likes too perch itself on the mast of tall ships. The drake is generally considered to be lucky by sailors and fishermen, and so (like the albatross) is not usually harmed. Since the drake normally prefers to take tiny fish, it normally doesn't attack sailors - even lookouts up in the crow's nest (called the drake's nest in my world) - at the time. That's a minor encounter, as is having dolphins leaping at your prow, or seeing a sea monster in the distance, or having a dragon fly over at a great height (but not be interested in the ship). All of this helps explain to the players what life is like at sea, what resources are out there, and frames later major encounters when NPC's of this category are involved.
After you do minor encounters for a journey or two, they can become assumed, and unmentioned unless the PC's specifically ask if they encounter something (in which case, check your table) or if they are potentially going to morph in to major encounters. Bring them in occasionally only to avoid alerting the PC's that every minor encounter is now no longer what it seems. However, you can turn minor encounters into major encounters by changing them up slightly. Sea gypsies might be bandits in the future. The next sea monster might be hungry. The next fishing fleet might be currently fighting off a sea monster. The next dolphins might be the pets of sea elves, who then act as sea gypsies or merchants. The next spire drake may attack the lookout, and the resulting death of the drake (or the lookout, or both), may be a source of consternation by the crew (and possibly real bad luck). The next plague ship may request aid from the party cleric, resulting in tension with the crew (which wants to avoid infection at all costs). The next plague ship may actually be filled with sentient undead, resulting in a fight with a undead 'pirates'. Or plague may break out aboard your own ship as a result of contaminated water or food or rats picked up at the last port of call. The next warship may actually be at war with the flag you are flying, and decide to attack you. The next prison galley might be from a rival nation, and the captain of your ship decides he wants to rescue the prisoners among whom he believes are prisoners or war or captured citizens of his own nation. The next customs official may be corrupt, and need a bribe, or there might be a person in the crew smuggling contraband and now the whole ship is suspect and subject to impounding if you don't smooth it over. And so forth.
Another thing you can do since you have a fantasy campaign is have fantasy ocean terrain. You can have a channel with crashing rocks, where some sailing knowledge is required to navigate this successfully. (The DC should be low enough that reasonably compotent NPC's - 3rd or 4th level Experts should basically never fail). You can have periodic fantasy hazards like giant whirlpools that are known to form at certain times of the year or times of day (again the point here is mainly setting color, not to be actual challenges to competent knowledgable sea farers though of course if the PC's are not and in charge it may motivate them to become so.). You can have wandering islands on the backs of giant sea turtles, or floating castles (or cities) held up by magic. You can have a Venice that actually is floating on great barges out at sea. You can have the ship run 'aground' in a mass of sargasso and have to be cut free. You can have a city of cloud giants which demand tribute lest they unleash storms or winds on the ships below.
So far, all I've got is monster and pirate encounters. But that could be no different than random encounters in land based travel. How do I make those things feel ocean-y?
Ocean travel is dominated by weather, and particularly dominated by the wind. You'll want to do some basic research and climate modeling to set prevailing winds, stormy seasons, and so forth. And you'll want to generate weather results on a daily basis.
The secret to making a campaign feel 'ocean-y' is solid rules for manuever and combat with ships. I would suggest one of two models:
1) Ram based galley combat, based on actual ancient galley technology of the bronze age era.
2) 'Cannon' based sailing ship combat, based on casual fantasy versimilitude and the actual real world technology of the great age of Sail (17th and 18th centuries especially). Cannons here can be actual blackpowder cannons, or they can be alchemical magical devices, or they can be mangonels other torsion engines where you make it realistic enough for a fantasy game. Our prior multiyear sea based campaign was based off this last assumption. One nice thing about using mangonels or ballista is that you keep the scale of combat in the range of 100's of yards or 100's of feet rather than in miles, which works better on a table top.
Those are the two models that will generate the most interesting tactical and strategic situations. From Rome to say the Spainish Armada, sailing combat is tactically uninteresting, being basically battles on land with semi-mobile castles. Fleets just sort of smashed together and then you fought it out over the resulting terrain. That can still occur if you adopt one of the two above models, but its only one option.
The advantage of having naval combat which doesn't wholly depend on boarding actions is precisely that it feels different than battle on land. A battle can be in part or whole decided by a whole different set of considerations than who swings a sword the best. You may want to introduce spells that balance the technology with available magic. For example, in a world where sailing ships are important, the ability to magically make cloth fire resistant on a large scale may be a prerequisite. So, you might want to introduce a 2nd level spell that can permenently make objects resist burning (at the cost of some amount of reagents and possibly XP). Warships can be presumed to have been treated in this way, and as such aren't completely dominated by fire magic.
