Did you use a new (or the old) paper gasket t oseal the cover on the pump.
If so, take it off, and try a bit of RTV silicone sealant (auto parts store). You want only a little sealant, as too much ( or a buildup of old paper gaskets, etc) will cause the cover to sit too far from the impeller, and it won't pull as much water.
Did you reuse the old cover? Was it scored?? If so, clean the other sit and flip it over.
I used to have the exact some trouble as you. Had to close the engine seacock while sailing, to prevent water from running out / air pulling in (maybe from the exhaust). Turned over scored cover to other side, and it;s been perfect.
You could also have an airleak in the new hose / clamps.
Sailing / water rushing past hull will cause (venturi effect, I believe) the water in the through hull and hoses to ne "pulled" out, if it can be replaced ny air squeezing by the pump / cover plate, oe anothre source (airleak in hoses)