About every two years with my boat on either my lift, or on my trailer, I will jack up the bow and tilt the boat to one side. Let it sit that way for several hours - or days if you have the time. Then remove the hatch for the fuel level sender (If the hatch for the fuel pick-up is more convenient, use that) and snake a small hose to the lowest corner of the tank, and pump out several gallons of whatever is there using an aftermarket 12 volt fuel pump. (A hand pump will work as long as it is rated for distillates.) I'll usually get anywhere from a quart to a gallon of water, gunk, and red/black grime.
As Lepke stated, bacteria will grow in the water. The water gets there because the tank is not sealed so you can get considerable condensation from the surrounding air. Plus, some marinas never drain their storage tanks, and don't have effective filters on their dispensing hoses, so you can get water in the fuel from there, also.
BTW - it's always a good idea to occasionally pull the fuel pickup tube out of the tank and give it a good inspection. Some have a screen on the end which can get clogged up. Also, most have an anti-siphon valve at the top where it screws into the tank. Make sure it is working. If not, replace it. In older boats, the pickup tube was plastic, which can be compromised my ethanol fuel. Make sure yours is aluminum.