If your tummy tuck was 3 weeks ago, and your vacation is in a few weeks, I'll guess that the timing of your vacation is at least 6 weeks from the time of surgery. Hopefully that is a valid assumption. If you were my patient, six weeks out from surgery, I would feel very comfortable with you getting in the ocean water and the pool. Assuming no postoperative healing complications, your incisions will be completely healed by that point.
If you are going to the beach, be sure to apply copious amounts of sunscreen to all incisions. Even though your incision will be covered by a bathing suit, ultraviolet rays can still penetrate your clothing and cause your scar to develop hyperpigmentation (dark color). Apply sunscreen every hour and re-apply every time you come out of the water.
Best of luck!