There are some things you can do on the suction side without pulling the boat out of the water. If you remove the hose going to the suction side of the pump, you can try blowing through it to see if there are any obstructions. If you are not able to blow hard enough to get bubbles to come out at the drive, you might be able to use a vacuum (with the hoses switched) or compressed air. This works well if there is a plastic bag or debris in the water passages.
Also, you can get in the water and inspect the outdrive to see if there is anything visible in the water intake area of the drive.
1975 Trojan F36 Convertible, Twin Chrysler 440's
2014 West Marine AL360 Inflatable, 1966 Mercury 6HP, 1992 Mercury 20HP