You can lose maybe 4 pounds of fat in 3 weeks if you are really diligent about it.
I'm no professional, but if I were you, I'd do some really intense cardio intervals. I started swimming recently and it gets my heart going like a freaking jackhammer, so maybe that'd be a good approach. Do as many laps of the crawl as you can manage at a high speed in a local pool, then rest for 3 minutes, then do it again, and repeat for 15 or 20 minutes. Do that 3 times each week.
Then do some weight work. Eat plenty of protein. Track your calorie intake using a tool like www.fitday.com. Figure out your resting metabolic rate, somehow (maybe just guesstimate.) To lose 4 pounds of fat in 3 weeks you'd have to eat a daily calorie deficit of 600 calories (i.e. consume 600 fewer calories each day than you burn.) If your RMR is 1900 calories that would mean surviving on 1300 calories a day. That's probably impossible without feeling like you're starving (and feeling hungry often is counterproductive, if you starve yourself your body will try harder not to burn the fat) so you'll need to augment with exercise just about every day in addition to those intervals and weight work. Walk or bike several miles every day to buy yourself another few hundred calories' worth of food so you feel satisfied.
Eat only whole grains, lean protein sources, and a ton of veggies.
Like I said, I'm not a professional, but that's what I'd do.
At 150 lbs, losing 4 lbs of fat could make a pretty serious difference in your body fat percentage. What is your body fat percentage, do you know? I can't guess very well since you don't have a photo up, but from your description, I'm going to guess, uh, let's say 18%, if you have "just enough fat covering your muscle." In that case, you'd be carryinf 27lbs of fat tissue on your frame; losing 4 of those pounds would drop you almost 2.5% in body fat, which will be noticeable.
Even if you can't achieve the 8% body fact, I'm-smuggling-anacondas-under-my-skin six-pack abs look, losing 3 or 4 pounds of fat in 3 weeks will likely be such a noticeable difference to you personally that it'll be a big self-confidence boost, and nothing's sexier than comfortable self-confidence.
Good luck, and enjoy that cruise!