I've known guys who put brass petcocks on the block between the knock sensor and block with no problem. One concern however is pieces of rust or other such crud in the block can easily clog the petcocks. Anyway, pretty straightforward...after a day on the lake, drain the block, drain the manifolds, pull the raw water hose, blow out or pump out the heater via a flushing T on the heater line. They claimed that took no more than 15 minutes.
'04 MariStar 230VRS/MCX