For the OP, I really enjoy using the mobile structure rules for making ships. You choose a size and height, and the type is abstracted to hangar, building, fortress. Hangar is mostly open, and has a cargo container ship as the example. It would also work good for oil rigs, which id imagine would be a huge part of a blue water world. A building is enclosed, so a cruise ship or fishing vessel where all the stuff is inside save the roof/deck. Fortress types are the ones with armor. If a ship has armor, its the fortress type.
The mobile structure has far fewer minor things to add for tonnage, so making lots of different ones isnt nearly as much a pain. Its just CF in place of tonnage, keeping it in a 20-100 range for most things per level. You can take the dimensions of a real vessel, and its speed, and draftx2 for levels and just choose the type that fits that length/height--and bam units done.
Edit: as a proof of concept, lets make a stand in for an LCS. Its 110ish meters, so I'm gonna go with 4 hexes long and 2 high. The littoral is a pretty small type, so lets go medium weight with a 40 CF. Its fast with a shallow draft, giving it 2 levels per hex to ensure the minimum draft for shallow waters, as half the total levels rounded down are above water.
We will do the max speed of 3, with a fission engine so no fuel. This takes 9 of the 40 CF. Add another 2 for controls. Finally armor, 40 armor per hex is 2.5 tons, divided by 2 levels. This leaves 27.75 x 8 for hexes and levels, or 222 tons for stuff. Lets do 4 light vee bays, for 2 vtols and 2 landing craft. With 22 tons remaining, that's 4 light and 2 medium machine guns, an ac5, 1 ton of ammo for each type of weapon, and an LRM5 with 24 shots in place of 22 rolling airframe missiles. The last 5 tons will go to infantry bays, or you can mount turrets for the ac5 and LRM5 for less infantry tonnage.
We have a serviceable littoral ship capable of operating in shallow water. Mobile structures have crew and such, but don't pay tonnage for it since its nice and abstract.
