Can I use Liquid Plumber to clean a clog in the shower drain on my 47 Fountain?

No. Most drain cleaners contain some type of acid. Could be sulfuric or hydrochloric. Very bad for the pipes. If you read the contents and it is truly acid free,it would be OK. Did you take the top off the drain and see if there is any thing clogging it right there? I would prefer using one of the drain cleaners that use compressed air. Or just make your own, by putting an air hose wrapped in a rag in the drain to seal it and slowly turning on the air. it should blow it out. Another way to do clean a boat drain is to take a wet/dry vac and stick it over the drain opening and sealing it as much as possible. Chlorine bleach sometimes will work also. Stay away from acid or basic drain, sodium hydroxide,(causes heat, warp plastic pipes) cleaners.