HalfLife:2 Lost Coast - Sailing 101.1) Choose your craft wisely...

(The metal blue boat is the best, as it can withstand gravity gun hits better, also the weight stops it from launching in the air with each hit)
2) Line up boat with destination.
3) Push off and jump...

(This is the tricky part, try not to land at the back of the boat or it will move backwards and beach again, however it you are too far forwards you cannotmove the boat. Ideal positioning is just in front of the middle)
5) Lodge yourself firmly in the boat.
(crouching is best: low centre of gravity)
6) Gravity Gun "Punt" your craft forwards...

(This is the fun bit, if you are lodged properly {i.e. not able to move AT ALL} then you'll be quite safe)
7) Repeat steps 5+6 til safely landed at destination...

(Under no circumstances fall in the water, there are "wormie" things in there that kill you v. quick!)