it says on the website that it can be used in pools, but I’m enquiring as to whether it’s safe to take it in salt water
Should be safe. Just take care to rinse it in fresh water after each use in sea water.
Yes, make sure you rinse it afterwards.
Also, note that the venu doesn't support open water swimming as a native activity. It does support pool swimming (laps) which is probably why you saw what you did