Your right, they are two seperate issuesand I'll adress them seperatly. First the shower and its inability to drain. Had the same problem only mine would not drain at all. Your are also right in that it is a clogging problem but its not the drain that is clogged (or at least i doubt it is). there is a line that runs from the drain under the floor and back into the cabinet below the head sink, where it joins the drain line from the sink to the through hull at a T connection. Somewhere between tne drain and the T connection is an inline filter about the size of a loonie. More than likely that is what's clogged. follow the line, find the filter and clean it out (it's easy to take apart, but hold your nose, its going to smell).
Second is draining the fridge. The fridge has a drian valve. agian it is a drian line that goes from a hole at the bottom of the fridge runs along the port side under the stove and kitchen counters to the port side sofa. lift up the cushions and agian there will be two wooden covers over two access hatches. Somewhere in that storage area you should find the end of the hose with a brass quarter turn valve on it. turn the valve and the fridge will drain. I keep a two litre pop bottle to drian it into, then dump it down the kitchen sink. Its not recommended you drian it directly into the bilge due to odour problems. If the last owner wae messing with the fridge my last directions may not be valid. However I cant imagine he would have messed with the means to drain the fridge.