My knowledge in this area is very limited but personally I wouldn't put an antenna on a fold down bimini. I went for a 6db 2.4 m fibreglass mounted on the side of the cabin, forget the reasons now but something to do with the way the signal is promulgated (cant think of a better word)like a horizontal figure 8 and even the rock and roll of the boat can affect your transmission/reception and (don't quote me) but I think a 20m antenna the effect can be worse.
My antenna is with a separate cable but from experience very few are well sealed in their connections so where the cable enters the fitting I've added extra sealant and even on the antenna itself added dual wall heatshrink to prevent any moisture entering as this affected my last one even though it looked perfect from the outside.
You can get aerials for your mobile that extend your range quite considerably which in your area may or may not work.
As you say you need some advice from your locals.