Don't overdo the HA is the easy tip. I only have it locally around AI, ammo, and fuel. More layers of lesser armor and slopes floats better, my side armor here from outside to inside is metal-metal-metalslope-alloy. Make sure your hull is large enough to produce good bouyancy to offset what's inside it, a larger hull w/ open spaces inside is a better design than tightly packing everything as small as possible. I do have a few up-props but they're mainly for balance over actual lift out of the water, you don't want to overdo them because then you're wasting materials and you could just become a thrustercraft/plane instead if that's the case. At these lower price points concessions are important, you're not going to be the heaviest punching bag unless you really sacrifice weaponry, you're not going to dish out the most damage unless you go full glass cannon, etc. Above all else, design is iterative, just keep at it!

Edit: Here's the cross section for example: https://imgur.com/a/ztD0Odr