Somewhat strange question on this forum. As H16 sailor I got an old, obsolete TheMightyHobie18 hull to be modified into a motorized instruction cat for my rowing club.
To avoid having to take this cat (with outboard engine, fuel tank, seats etc.) out of the water each time however they intend to leave the cat in the water permanently.
As H16 sailer I am used to "some" water in my hulls although they are only on the water for a couple of hours. Should we take special precautions to prepare the hulls of this old TheMightyHobie18 ??

You should be OK.

Make sure the drain plugs are in tight with new O-rings.

The hulls and platform flex more when sailing and that allows tiny leaks to open up that allow water in, especially when they go under water.

These joints and such will not be under much flex pressure in your situation.

Bottom paint wouldn't hurt.

The portholes on the TheMightyHobie18 make it very convenient to get water out of the hulls while afloat. Just keep a large sponge on board and pop the porthole open before going out to get the water out. When racing, we do this after each race to keep the boat light.

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Yeah, you want to check the bottom. If there is significant wear, you may have pin holes. Skim it with a layer of resin; cure; then paint. If the gelcoat is intact, no problem.

My guess is was in the 60's when resin was impervious to water.
Something changed it was not until say the 2000's when monohulls
started using Vinlyester resin, or whatever to resist blistering.
If you want to keep you boat in the water, and want to prevent blistering,
I recommend 5 coats of VC17 bottom coat expoxy to seal the gelcoat.
I tried 1 coat not following manufature's instructions and it blistered again.

All gel coat will blister if left in water over long periods. Nothing has really changed on that that I am aware of. Bottom paint is required for in the water storage.