I would think a Check valve would create a restriction, those passages are too small.
Intercooler wants at least 20 psi and lots of volume of water.
I have a -16an feeding mine and two -12an lines in the back that go overboard.
Good volume without creating extra pressure.
I run mine full time of a 3 stage pump , one stage dedicated to intercooler.
Im not seeing much of a difference between running cold water thru full time or only at speed like my last set up except way cooler temps at idle .
Idling my IAT`s would climb to 200* in the LOTO heat .. I was way behind on the cig run because the intake temps got too hot idling waiting for everyone and my guardian mode set in . Now it`s always below 100*

So I'm going to have to go thru the bottom of the boat and not the transom for the pick ups. Anyone know how close to the center of the hull I can go? I've for other thru hull fittings I'm trying to stay away from.