Not sure I agree with the "get 18 in your prime stat easily", but I'll let it go.
I'm also not sure you can stack multiple Synergy bonuses to a skill. Even so, if someone is willing to spend 20+ skill points by 2nd level to get one thing, then that's probably the only thing they'll be very good at. at Diplomact is easy for a DM to shut down, if it's being abused: A Diplomacy check takes one minute (10 rounds) to make. If the opponent is hostile, as in, going to attack NOW, Diplomacy doesn't apply. It can't be done in time.
Still, I agree that Diplomacy and Intimidate both need some work. They both operate with a fixed DC system, as opposed to being an opposed roll. Bargaining and persuasion are one-sided conversations per the rules, and clearly they shouldn't be.
As for the questions, "How does 3e simulate" my Ranger question: One rank in Profession - Sailor" does the job. Simple knots, shipboard terms. Also, at the DM's option, if he/she already has Knowledge - Local, it might suffice.
Knowledge - Local is one of the least clearly defined skills in the game. We interpret it as "You are good at getting the lay of the land, socially speaking." So you might not know the good inns, by name, but you can find them by looking at the neighborhoods. You can pick up on the local slang and customs. I mean Gather Information is nice, but finding where the needed information might be found is good too. You may not have ever been to this town before, but you can 'sus out who's in charge and who to avoid. In other systems it might be called Streewise.
Many complain about Profession as a skill as well. It's actually pretty clear in the rules: If it's the ability to make (and sell) goods, it's Craft. If it's the ability to provide a service, it's Profession. So Craft - Shipwright is wrong (nobody builds and sells ships by themselves). Craft - woodworking is correct.
I've heard some argue that everybody should have Spot and Listen in class. I don't agree. While anyone can make a Spot roll (Wisdom based skill), relegating the masterful training to Rogues, Bards and Rangers makes a certain amount of sense, in terms of game balance: All of those classes are skill-monkeys because they need the extra skills to raise their in-game utility to the point where they can keep up with the A listers.
None of them can wear the "good" armor, and none of them gets anything big in terms of magic. Rangers can have decent hit points, but their major combat styles (two weapon fighting or archery) both make shields useless. Bards can't use them at all.
Rogues, the kings of the Skill Monkeys, ironically find themselves short of skills. That is, while they get a lot of points ( nine plus race plus INT modifier), INT isn't usually one of the stats they pump. Additionally, they have a LOT of things they need to be good at: Spot, Listen, Hide, Move Silent, Search (for traps), Disable Device, and Open Locks are all on the "gotta have them" liist. And that leaves them with two they can use optionally. Just like the Wizard, Fighter, Sorcerer, Paladin, and so on. Climb, Jump and Swim are on the "it's nice to be able to do it" list for those optional skills.
As for Skill Checks. 15 wasn't considered "hard". For may things it's the minimum for basic success: Heal check? Roll 15 to staunch bleeding and/or clean a wound, or care for an injured friend to accelrate their natural healing speed.
Want to Climb a rope? DC 10. Knotted rope? DC 5. stay on a horse's back while walking? DC 5. At a Hustle? DC 10. Not in combat, of course.
Open Locks? Those start at a DC 20 and go up: If it was lower then anyone could Take 20 and open most locks.
Simple Spot and Listen checks? How simple? Hear a nearby commotion? DC 0. Be awakened by a nearby commotion (attack on the camp)? DC 0, but with a 10 point penalty for being asleep, so DC 10. See a mountain in the distance? DC 0. See details on that mountain, such as trails or buildings, from a good distance? Start at DC 15 and go up, depending on circumstance.
Want to swim? DC 10 is enough to keep your head up.
So "simple" and "tough" skill checks vary, depending on the skill. Why? Because IRL some things are just harder to do than others.
Over all, though, I favor the granularity and flexibility of the system, traits the later editions aren't exactly known for.