Spar varnish is called a "long oil" varnish. Varnish is a resin cooked with a drying oil. Typically the resins are urethane (or polyurethane), alkyd, or phenolic (and some others). The drying oil is typically linseed oil, tung oil, soya oil, and a handful of other. What makes a "long oil" varnish is a higher level of oil to the resins, this makes the finish more flexible for wood movement. That said, I still wouldn't use a spar varnish in the bath. A good quality alkyd or phenolic would hold up very well. Waterlox Original is a phenolic/tung oil, and would really serve you well. A couple of others would be Sherwin Williams Fast Dry Oil Varnish (alkyd/linseed oil) or even Pratt and Lambert 38 (alkyd/soya oil). Waterlox is likely the most durable for your use.