sealing larger openings into the below floor area would seem a good idea as an attempt to keep mice out, but I wouldn't try to seal it up against water - I think whatever you do, water will find its way in and if the floor or the belly pan is sealed with tape, it will just lay there causing very humid conditions and corrosion, If anything, as Tim has done, I think a few strategically placed small drain holes would be a good idea!
Brian & Connie Mitchell
2005 Classic 30'
Hensley Arrow / Centramatics
2008 GMC Sierra SLT 2500HD,4x4,Crew Cab, Diesel, Leer cap.