I sometimes forget to turn off the battery switch for weeks and have no problems. My battery is much larger than yours though, probably.
Does your ski have a bilge pump? Some of those things turn themselves on automatically about every 3 seconds or so, stupidest design ever, IMO. Maybe okay for recreational purposes only and great for battery salesmen getting kickbacks on bilge pumps.
Anyway, could be leaky charging rectifier or even that surge suppressor buried in most MPEM's might be leaking. Wire an ammeter in series with the battery then start yanking connectors out of the circuit till the drain disappears.
Most all batteries are crap anymore, if one cell doesn't s**t the bed the first year then the internal swaged lead connectors loosen and corrode so use a backing wrench when tightening the the fasteners so as not to put stress on the terminals. Prorated warranties are for a reason.
Batteries by mail? I don't think I've ever received a package shipped through UPS or FED-EX that wasn't crushed in transit, even those guys unloading batteries off the Interstate-Battery truck throw those things around and stack them on top of each other which can turn them into nice door stops b/c of the poor construction methods.
