As fasthawk6 stated, might just need to prime the system so it draws water. The water pickup is quite a distance from the pump and is an uphill climb.

I don't have a Sunsation but on my Velocity the a/c will run for 15-30 seconds (can't remember) and stop running if there is no water flow. I just turn it off and on a few times till the sea water pump gets suction and starts pumping water. I just look at the outside discharge fitting for water flow and know it's working. Sometimes it take 2-3 cycles to start pumping.