As06655 wrote:Q1: how do you get the water out [of the boat after it flows out of the fuel tank cavity via the new drain tube]?
Water is never going to flow uphill by itself, so a pump must be employed to lift the water to some place where the water can be moved off the boat.
As06655 wrote:Q2: [can a] Shop-type [WET-DRY] vacuum [be used to remove water that drains out of a new drain installed in the fuel tank cavity of classic-ear Boston Whaler boats with a below deck fuel tank]?
Yes. If you use a shop-type canister vacuum the water will flow into the canister of the vacuum.
Most shop-type canister vacuums run from 120-VAC power. On smaller classic-era Boston Whaler boats with integral below deck fuel tanks, there is generally no 120-VAC power available on the boat, often even if tied to a dock with shore power. Your proposal to use a shop-type canister vacuum would only be practical if there were plenty of 120-VAC power available. Many shop vacuum electric motors draw 10-Amperes or more current when operating.
A vacuum pump can only lift water a finite amount, depending on how much reduction in air pressure it can create on the suction side of its filter media. A vacuum pump would need to be tested to see if it can lift water out of the bottom of whatever part of the boat you propose to have the fuel tank cavity drain into.
Typically in a marine application a pump that will be used to lift water into the pump itself will be a bellow-action pump often called a diaphragm pump. Diaphragm pumps are much more expensive than small centrifugal pumps, but a good diaphragm pump can self prime by lifting water a foot or more, and can tolerate a much higher height of head in the exhaust path.
A centrifugal pump generally has zero lift and a very limited range of head on the pump at which height it can produce any significant flow of water above the pump intake. A centrifugal pump inlet must always be completely submerged by water for the pump to produce any flow at all. This results in there always being residual water left in the sump.