We've done this several ways and it depended on the boat we were on.

For the no TP toilets we had sandwich size zip top bags for poo and those were sealed and put in waste bin lined with trash bag. TP used for pee only was just put in trash bin. Throw out trash bag as needed.

On another boat where we could flush TP we "threatened" our crew if they got the toilet clogged they were responsible for full payment to get it unclogged. They should use as little paper as possible. Well, at the end of the trip (of 10 days) I realized a 4 pack for one of the cabins had 3 rolls left from the 2 people. I asked why so much left and we had scared them so bad they were afraid to use much at all. Used about 1 square per usage. 😂😂😂 We really didn't mean THAT little but kudos to them. Now we have great stories for life!

Use bathrooms on shore as much as possible also helps.

Good luck!