The bubbles are created by back pressure from something that's not allowing air displace by the flush to escape downstream of it. That can be a blocked or partially blocked tank vent or seawater mineral buildup in the discharge hose that's seriously reduced its diameter (I've seen it reduce a 1.5" hose to <.5"). Mineral buildup is a lot higher in warm waters than in colder waters, so it clog hoses pretty quickly in hoses on boats in warm water
So check your tank vent for a blockage...the most common locations are the vent thru-hull and the other end of the vent line, both that end of the hose and the vent fitting on the tank...use a screwdriver blade, icepick--whatever works to scrape 'em out and clean out the toilet discharge hose using about 15% solution of muriatic acid (available from any hardware
store) in water. Rinse after about 45 minutes with clean FRESH water. Repeat if necessary. Muriatic acid is nasty stuff, so be sure to CAREFULLY follow ALL directions for safe handling and use!
The joker valve will almost certainly be encrusted, so you'll have to replace it too.