Never tried a swimfeeder in the sea , but I don't see why it won't work , how you use one from a boat I don't know , the skill of using a feeder in fresh water lies in being able to cast it to the same spot every time so that a concentration of food and smells builds up in that area.
From a boat it's going to be difficult to hit the same place every time.
As for what to put in it , you want whatever you are using for hookbait but minced into fine particles , it puts lots of smell of your bait in the water .
Don't do what I watched John Wilson do on one of his programmes , he was shark fishing with a rubby dubby bag full of minced mackerel over the side of the boat, a big blue came up behind the boat and was obviously attracted by the smell , so Wilson and the rest of the anglers on the boat dropped a hook over the side baited with a side of tuna

, which despite being wafted back and forth in front of the sharks nose , it totally ignored.
Soon as someone finally clicked and put a rotten mackerel on, the fish was hooked............. about 5ft away from the boat and classed as landed because someone touched the leader (which hadn't got wet yet)

Buy some and try them
Ray