
I too have some Paramount convertibles and I can convert them wearing my size 10 Scarpa SLs with no problem, so I reckon 11.5s could be ok.
They're quick drying and are a fairly sturdy fabric so dont immediately get soaking wet, though they are by no means water-resistant really (though neither are the Montanes you mentioned).
The only gripe I have with them is that they have quite long legs. Im 5ft 9ish, 32 waist and wear a medium, short leg, which is fine at the waist but the legs nearly drag along the floor.