The lower unit where that hole is is all aluminum and will clog up (especially in salt water if not flushed regularly).

The rubber hose (that carries the water pressure) connects to a longer hose approx 2' out from the engine and connects to a longer hose to your gauges. You can try flushing from there with the help of an air compressor. Depending on your engine, there may be a connector right at the lower unit and mid section but it'll be hard to get to without dropping the lower unit..

As a side note, some members have put a drill bit in that hole to clear it and have busted the bit in the hole (not good). If trying this, simply rotate the bite with your fingers, NO DRILL MOTOR, blow and flush as you do this.

The below link is a a set up I made on my boat to keep it clear (and works very well).

http://www.yamahaoutboardparts.com/f...g-th23159.html

Please post how it works out for you...