Speedos .. or perhaps something a little less disturbing! And NO not lycra shorts italian style.
Nothing worse than a full grown man walking around a boat in Speedos Drew. Boardshorts don't weigh much at all, I'm sure you could fit some in

Well, avoiding the fashion and cultural implications (think Brazil and France
), there's also a utilitarian component you prudish types don't seem to accept. Excess material creates folds and creases under the wetsuit, making it sometimes uncomfortable (for me anyhow). Weightwise, 3 pairs of Sungas (brief swimwear) weigh less than 1 board shorts, and will dry quicker. On a boat, on a trip with the sole purpose of diving, speedos are just the most practical swimwear to use. And I also don't care what other people think of what I'm wearing. 
That said, if I were surfing, kite surfing, kayaking or any other vigorous activity , I wouldn't wear a speedo cos they'd rip apart. But under a wetsuit, comfort is the most important thing for any activity. And a speedo is that for me.
I'm not sure where Tim is going for his trip. But if he had to pack for a winter/cold water dive trip (say Antarctica or Artic) then 55kg would be a great weight to have.