I have been to most of the world's top aquariums including Okinawa, Genoa, Sydney, Valencia and Dubai.
I felt that Underwater World wasn't on par with any of those... but it did fill in a day in our holiday when the weather wasn't so great and we decided to do something other than go to the beach.
The aquarium is conveniently located near several of the hotels and shops around Tumon Bay, so we walked from the Westin and were there in under 10 minutes.
The aquarium's tunnel is apparently the longest in the world, although it didn't actually feel especially long compared to some others we have visited. Overall the tunnel and general facilities felt a bit tired, but some parts were undergoing maintenance so hopefully will be refreshed soon.
We visited on a weekday outside of Japan holiday times and there was almost no-one else there - which is at least one area where this aquarium was good - we didn't have to battle crowds or feel rushed.
It turns out they have reasonably good wifi too, which is handy since I needed to get some work done while I was there and it saved me using roaming on my phone ;)
Our kids still enjoyed this visit overall. Especially if you haven't been able to see other great aquariums, Underwater World still has some fine sharks, rays, sea horses, reef fish etc etc to enjoy.
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