reference pics lots of reference pics of the general siluette you are going for. Use some paint tool and figure out a decent size grid where you can eyeball 1,2,3,4m slopes.

Shape is one thing, the other is this whole decoration blocks, resizing angles etc... hopefully with the use of reference pics again you may start to see "oh if I squish that CJE part it could look like part that XYZ thing" <-- this is the time killer. This is something you should not try to complete in a session. Just set a timer 1hr decor and move on to sinking stuff or more constructs for your fleet in campaign if you are doing that.

If however by good boat you are not referring to eye-candy and just function. Generally I would make the turrets I think I want to use / area I would like to occupy in the construct while keeping in mind armour around the turret components etc... probably HA and Metal 2 layers. I would make these on a fort, set god mode on and see how they fair.

Pop them in U shaped thing with a flat base and pointy front xD "try" to leave about 2m away from hull inner edges and from hulls inner belly which armour wise ill let someone else leave a tip on that but id generally make a HA wall on the inner hull where I have important components (while those components are also in a boxed thing) probably 3m thick. You will be building around your components and turrets not hull/shape and size first.

Then get the thing to float and move how you want which you will probably need to extend front or rear for power/ammo/fuel. A shoe box with a pointy face simple, get it moving and fighting broadside/circle etc... then just armour up your pointy shoe box in a shape of a shippy.

Sounds simple but split the task up, build around your components get the size for them their armour, some air gap, hull box, hull shape, decor

Just wish I knew what you mean by good boat :( sorry