Sealant...definitely a good thing. Also, testing in shallows/water that's not bad to wade in.

My shallow test site is great for retrieval, but it's so full of weeds it stops the subs more often than open water does. I've retrieved a sub a half-dozen times in a session to untangle the prop.

Yes, my last sub was lost in a little lake. Town Lake is the south border of downtown here. Max depth is 18', average 14'. I saw the light on the sub go down deep before I lost control--but I wouldn't doubt a weed ate it (which would explain why I couldn't see anything anymore).

http://southwestpaddler.com/docs/colorado3.html
There's a photo "Town Lake from the mouth of Barton Creek"--picture me rowing in the middle of the lake, and just out-of-frame to the right--that's where I lost the sub.

Often weeds are a silent killer in this hobby. I think I also lost a brand-new sub to weeds once--but I'll never know for sure !