It looks like the automatic bilge siphoning tube with the screen end in the bilge. Check the Mercury manual Section 1D Mercury Jet Instalation.
The Sport Jet incorporates an automatic bilge siphoning feature. The bilge siphon is working whenever the engine is running above idle speeds. Maximum performance of the bilge siphon is realized above 3,000 rpm. A hose is attached to the jet pump nozzle. The hose is routed to the engine compartment and placed in the bilge. Water exiting the nozzle creates a suction or vacuum in the hose creating the bilge siphon, drawing water out of the boat.
The siphon break must be located above the water line at the highest point (sta-strap). The siphon break has a 0.020 in. hole which must be kept open.