1) Patrons pay passage to nearest port, PCs board ship and sail there.
2) PCs take Patron-provided sack of gold, and purchase a keelboat with sail, a cart, and other supplies. Better yet, they are met, as they disembark the ship, by an agent of the Patrons whom they have perviously met! He escorts them to their supplies, per-loaded aboard the boot/cart combo, and wishes them well!
3) PCs sail to the point that they have to portage the boat, pull it ashore, pile unneeded goods/supllies into it, load it aboard the cart, and push/pull it, until encountering something.
You can call it a typical cart, as in the PHB, or call it a specially-designed "boat trailer", or whatever. Just make sure it has large, iron-bound wagon-type wheels, and good ground clearance.
Portaging a rowboat isn't really all that hard. Even the Hobbits did it, in the LotR! A rowboat with a keel (or just a rudder) and a sail shouldn't be any harder. The wheels just make penetrating the off-road terrain that I assume you have prepared for them a bit easier... they won't have to actually CARRY the dumb thing!
Supplies will be their problem... and (perhaps) overcoming difficult terrain! They are ALL going to need knives, handaxes, woodman's axes, timber saws, shovels, etc., for cutting through the brush in any woods. They will also need enough food to get there AND BACK AGAIN, unless one of them has a whole lot of Survival skill, and they don't mind travelling at HALF walking speed, while he hunts. And, of course, if HE dies...!
Do they have a Bag of Holding, or anything? Or do your islands not have a lot of hard-to-penetrate terrain? The tools they could carry in the cart (or affixed to the outsides of it, like Pioneers' wagons). They could load the cart/trailer down, quite a bit, since they'll be rolling the weight, instead of carrying it. Bringing their own food will mean they can travel faster, but if cleaning out the tower doesn't have to be done, right away, they could take their time, too. Whatever you want...
Besides overcoming the obstacles, they need to have enough gear to clean out the tower, and make it back, too! They will need tents along the way, bedrolls & blankets, extra ammunition (or tools and Crafting skills), torches, etc. If they are out of spell components & ammo by the time they reach the tower, they won't be much use! Their employers should "make arrangements" for them, and have them met with an appropriate amount of supplies, once they get off the ship in the nearest port. Some magical healing supplies (as well as typical Healer's Kits) should also be there, depending upon what each PC can use (potions for the Fighters, scrolls for the Cleric/Druid/Paladin/Ranger).