I'm going to disagree. Normally, I would say yes, get the Watkins part. I would even offer to sell it to you and ship it. But the fact of the matter is, it's only a "load center," or outdoor sub panel. Any one will work, as long as it has room for the right number of breakers. And since you have a GFI protecting the entire sub panel, no, you don't really need the two breakers inside the sub panel to be GFI breakers.
Save some cash. Be sure you find out if you need both breakers to be two-pole, or if you have the 'convertible' model Bengal which needs a single pole 20 Amp breaker and a two pole 30 Amp breaker. Then see if you want to put in another breaker for a GFI-protected outlet near the spa. Code requires it. Check with your local inspectors to see just how close/far away, but for most it is between 7 and 20 feet from the water.
However, I bet if you asked other electricians what the VERY SAFEST way to install it - they would all come up with different answers!! :-)