DH and DD7 did this in July and DD LOVED it. DH said there wasn't a tremendous amount to see, but it was neat. The ride out to the sub is rather choppy - again, DD LOVED it! One of the major highlights of the trip for her. (I went jewelry shopping instead!)
A few weeks later, DD's 8 year old friend went on the same excursion and was sick from the ride. Luckily it was BEFORE she got on the sub, so they went back and Disney did refund their money. If anyone is prone to seasickness, maybe a preventive dose of Dramamine would be a wise move!