I get water during vigorous port tacks due to my bilge pump being
tied into the sink drain.
On a port tack, the sink hose to bilge might be down hill, a check
valve is needed and a vent loop with a siphon break is a good idea
(in case the valve fails, which happens often, a little crap from the
bilge can keep it open, luckily everything is fine when the boat is
My bilge pump is also tied into the sink drain, but a check valve or
vented loop will let the water out but not let it in. You can get
them at most plumbing or hardware stores.