In the naval movement post, I already covered the basics of movement in a galley, both for sailing speed and rowing speed. A galley's standard oared speed forward is equivalent to its speed in a Light Breeze - 5 and causes total fatigue in the rowers after 10 turns. They must rest three turns for every one turn of standard speed. Slow speed is -5 and requires two turns of rest for every one turn of rowing; rowers can sustain slow speed 50% longer than standard. Fast speed is +10 and requires 10 turns of rest for every turn of rowing; it can be sustained for a maximum of 3 turns. Backing oars slows down the galley, or moves it backwards; back slow and back standard are at half the forward speeds, but there is no "back fast". Rowers can bring a galley up to speed at a rate of +5 move every turn, or slow it down at double that rate.
A Strong Breeze kicks up waves high enough to cause galleys to risk shipping water; make an Avoid Danger roll every turn or lose 1/4th speed. Add +1 difficulty for a gale. Once a galley has lost 3/4ths of its speed, it sinks. The open sea will also cause galleys (but not Viking longships) to ship water.
The same rules apply more or less to smaller boats and rafts. A raft has only one rowing speed, equivalent to the slow speed for a large galley (Move 10.) This is also its speed with a small sail, when running before the wind; rafts aren't built for effective sailing, so if the wind is from behind at a small angle, speed is reduced to 6, and if the wind is from the side or front, the raft must be rowed. A small or large boat has a slow rowing speed that's the same as a raft, but can be rowed at +5 Move for standard speed or +10 for fast speed; if equiped with a sail, it maneuvers like a small galley, but at -10 to all speeds, or -15 in a Strong Breeze. Rafts and boats ship water in open sea or strong winds as do galleys, and suffer the same problems in river rapids as well.
Small galleys have 50 oarsmen, with twice than on large galleys. Having fewer oarsmen means proportionally slower speed; generally, a ship has to lose 3 or four oarsmen before it loses speed.