Quite right. I had forgotten that. But best practice dictates circuit breakers limiting current to the rating of the receptacle and wiring. So installing a 15a rated GFCI outlet on a 20a circuit is generally a bad practice, and putting it on a 30a protected circuit is a very bad practice.
My solution was to start with an old 30a shore power cord with a burnt boat end. I cut off the bad connector, and attached a small 30a distribution panel with 15a current protected gfci receptacles. With this arrangement I powered two bubblers all winter. The panel stayed under cover in the cockpit of the boat. Photo attached for reference.
Again, I question the need for more than 15a of current protection.