The Wadadli Cat is well worth the expense. The resorts all warn you against signing on to "non-approved" excursions, but that is probably because the "non-approved" operators do not always provide them the commission they demand for recommending them and providing desk space in the hotel lobby for selling tours. 

The crew is great, the scenery unbelievable, and the Cat is spacious and comfortable.  Recommended is the circum-navigation tour of the island, and for those brave souls who enjoy a thrill ride, you should sit on the foredeck netting while the Wadadli passes from the Caribbean into the Atlantic and back, for about 20 minutes rounding the southeast corner of the island.  During this short turn through the Atlantic, the water becomes darker, and noticeably cooler, the wind comes up, the waves grow in height and it is almost a bit scary.  However, the Wadadli is completely stable and one never feels at risk during this time.  The bow dips into the ocean, bobs back up, and thrusts down again over each wave.  Those who sit on the bow will be drenched and it is like riding an ocean rollercoaster!!!  With the hot sun, it will feel unbelievable.

Do not visit Antigua without taking a Wadadli cruise.  You'll regret it if you don't.