We finally got a chance to check out Sea Life at the Mall of America (formerly known as Underwater World).
They host Homeschool Days once or twice a year, generally in the fall and spring. If you visit during those days (generally a week long), the price is drastically reduced. When we visited, it was $5 a person instead of $17 or $23 each!
The kids really enjoyed it, and they learned quite a lot. We made the three hour drive up there mostly for this event, and it was well worth it.
That said, I would not recommend it at full price, especially for large families or those with limited funds. While it was a fun visit, it would have cost well over $100 for our family and we were ready to leave in under two hours.
For the price we paid ($30 for all seven of us, since three year-old Fiona was free, I believe), it was a fantastic field trip!
We made sure to make some stops at places like the LEGO store while we were at the mall, too, of course!