I don't know if it is the MOST corrosive, but seawater, especially hot seawater, is bad news for metals, "stainless" included. It's not called "stainproof" after all. Having said that, there are less corrosion-prone and more corrosion-prone stainless steels. "316" is highly corrosion-resistant, but spendy. "15-5" is used for propellers, and maybe impellers. Try to find out what alloy of your impeller is made from.

I think your biggest challenge would be comfort, what with the almost ever-present 1'-2' seas and exposed conditions. If your hull is close to flat, it's like smacking each wave with a great big spatula. It's OK for a little while, but after 1/2 an hour probably gets pretty nerve-wracking. I know it sounds nerve-wracking when a sled goes by me on the saltwater Smack, smack, smack.

Regards.

Finegrain
Woodinville