So, it turns out that bats are actually pretty good swimmers; several species of bats have been reported swimming, and they're far more capable and agile in the water than they are on land:
If you think about it, some of the bats' unique evolutionary features could be extremely useful in an aquatic lifestyle. They already possess echolocation which is arguably more refined than, and superior to, that of cetaceans. Bats also have the most efficient respiratory and circulatory systems of any mammals, and their wings enable them to manoeuvre more accurately than birds, and fly more efficiently, with less drag and less energy loss.
Bats' Merkel cells also have tactile hairs which allow them to detect and adapt to changing airflow, enabling them to judge the most efficient speeds to fly at, helping them to perform complex manoeuvres to capture prey in flight, and even helping them to sense prey from the disturbances in the air. In the water, Merkel cells could potentially be even more important and advantageous. Shallow torpor and heterothermy would also be extremely useful.
And a full aquatic lifestyle would also remedy many of the bats' greatest weaknesses and limiting factors, such as most bats' inability to walk on land, along with their increased levels of respiratory and cutaneous evaporative water loss, which would cease to be an issue in an aquatic environment. Bats have already lost their sweat glands on their wings, and require more fluid intake relative to their body size than any other mammals (including cetaceans), making them highly susceptible to blood urea poisoning if they don't receive enough fluid. The most common cause of death for bats is dehyration, not predation; something that fully aquatic bats wouldn't have to worry about.
So, how plausible do you reckon it'd be for a lineage of bats in the future (perhaps the Noctilionidae, since they're the world's most amphibious bats at the present time) to evolve to become fully aquatic to the same degree as cetaceans, spending the entirety of their lives in the water and not returning to land at any developmental stage? What do you think that these aquatic bats would look like, and how large could they potentially get- would it be feasible for them to reach or surpass the size of manta rays? And what's the shortest timeframe that you feel such a lineage could plausibly evolve within, from the present day- 5 million years from now? 10, 20, 50?