Mike,
I learnt as I went on. Started out with a Billings kit - Bankert, which is the only kit I ever did. Then tried a planing hull (MTB Brave Borderer) and after that various types of model - yachts, fishing schooner, riva, etc.
You have started off well with speedboats, its a great way to develop the skills needed for more ambitious projects. When you decide to try something different, you will be surprised how much you already know. If you want to try something a bit more ambitious I would suggest you try a plank on frame hull next, maybe a fishing boat - the hull form is relatively straightforward, particularly if you choose one with a transom.
"Are you people real ship builders or something?" - not necessarily, although I think many of the blokes have a marine background, which does help in understanding what is right.
Ian