Amazon river dolphins could be a possabilite (if they import some), as it would go with the amazon themed area. good point Tigerlemurguy, manatees would fit in there aswell.
as bongorob said, there is no mention of any of them for biota! but (ading sealions aswell) commersons and atlantic bottlenose dolphins would fit with the atlantic, also pantagonian sealion and west indian manatee,
side stiped dolphins for the indian ocean, calafornian sealions for the pacific with common dolphin, and amazon manatee with amazo river dolphin in the amazon biome
river dophins NO, if you actually took the time to look at biotas website you will see what is included in the plans, i believe that manatees are included in the plans for the south american area.