Hi john
I was scanning though the explorer forum on the Findmall site about 4 or 6 months ago, and saw a post about a fellow doing just that! He had pictures of it so you can see how it was done. From memory, I think that he got the materials from a diving shop. Apparently the materials are fairly easy to come by. It want do your warranty any favours though. I'm sorry that I can't tell you the name of the post or even when it was posted, but if you go up and click on the search window and type in some key words like underwater explorer, or waterproofing the explorer eta, then it should bring up the post.
Who knows, you could try it with the Ace250!
Good luck,
Mick Evans.