For insurance reasons (and cost reasons), you probably won't find a twin otter available for time building. Twin engine turboprop... goodluck. Maybe someone can prove me wrong.
Calculations for a twin otter operating costs run over $1000 per flight hour, and that is for a land twotter. Seaplanes cost even more to maintain and operate.
I know a place where you can get training on a Grumman Widgeon for $500 an hour.