I always use a sheet of VisQueen (Plastic sheeting) under my tarps. Never have any water intrusion and each sheet of the plastic lasts several years since it gets no UV sunlight to degrade it. Comes in a 20 x 100 foot roll and you just cut off 20 feet at a time so it lasts for years and years.
https://www.ebay.com/itm/Clear-Plast...!-1:rk:13:pf:0
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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