I'd reccomend giving Haslar Sea School or Stormforce Coaching a ring (not that I'm biased of course
). They both run sea survival courses from the same location on the River Itchen, and have plenty of dates.
The Sea Survival is a really fun day, which makes a lot of valid points about abandoning to liferafts, attracting attention in Distress, and movement ability in Wet Weather Gear and Lifejackets. The highlight of the day is the Pool session, during which you practice getting in and out of liferafts (not as easy as it sounds!), working as a group in the water, experience being in an inverted raft, and practice righting a raft.
You'll have a good day, and take loads away with you!