it is probably the anti siphon valve on the seawater intake ,fitted between the impellor pump
and the heat exchanger
,ie seal, on positive pressure when the pump
is pumping,but allow air to enter when the engine is stopped to break any siphon and avoid filling the engine with sea water
the valve which will be located near the top of the engine compartment will be V,U or T shaped with the 2 seawater pipes from pump and heat exchanger
attached and the clear hose at the top.
the valve should have at the top where the clear hose is attached a threaded fitting which can be unscrewed,with either a plastic ball inside or a spring and insert with a seal.
you can dismantle and clean surfaces inside so the ball or insert seals
properly to fix the problem.
other wise if not leaking too much let the bilge pump
handle the water and let the charter
know that the anit siphon valve is leaking at the end of your charter
image here of a generic device,that come in a few different shapes and sizes for yachts,some plastic.