Wetsuits and vests are provided for all guests at no charge

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You need to wear either a wetsuit or a vest while in the water attractions (Grand Reef, dolphin swim, lazy river, and Freshwater Oasis). The Grand Reef is one of the cooler waters at the park (warmer than the dolphin pool, but colder than the Freshwater Oasis and the lazy river) so you will likely want the wetsuits there unless you are visiting in the summer months when the air temperatures are much warmer.
The wetsuits they were distributing on January 1st, 2012 had t-shirt style sleeves and a knee-length leg. But I have read from other Disers that full body wetsuits (with longer sleeves and a full leg length) are also available on colder days. I personally can't confirm that though.