Is it ok to have a 15 amp 240v outlet in close proximity to a sink?
GFCI
If the receptacle was installed many years ago, it might not have GFCI. If so, I highly recommend GFCI protection, even if not strictly required by code. (Code requires GFCI for any receptacles near a sink, but doesn't typically force retrofit if no changes are being made.) GFCI will provide the necessary protection. A weatherproof cover would help, but would not give you the life-safety protection of GFCI.
However, GFCI for 240V is not so easy. Very likely you would have to put GFCI at the panel. As an alternative, you may want to turn this into a generally more useful 120V receptacle (with GFCI). If the existing wiring includes neutral then you could actually turn this into an MWBC and get two separate 120V circuits out of it. If it doesn't have neutral then you can turn it into one 120V circuit.
Plumbing inside the wall and toilet on the other side of the wall should not be an issue at all. The concern about water and electricity isn't that all of a sudden your pipes will leak and get your wires wet. That can happen, but it is relatively rare - and usually part of a much bigger problem. The concern, and the reason for GFCI, is that people using water will touch something electrical and end up (through a combination of events - not every touch of everything electrical when wet is actually a problem) with some electricity flowing through people instead of wires. GFCI detects that very quickly and saves lives.