"mummy will come back when you stop crying"
I hate that mentality. My FIL tried to pull the same thing with me when he took DD out of my arms for a cuddle, and wouldn't give her back until she stopped crying. Needless to say it didn't go down well, and FIL soon learnt what it was like to have something snatched out of his arms.

Anywho...

Like the above, could you get in the water with her until she feels safer? Or at least sit on the edge and be close by? If they don't allow it with the swimming lesson, perhaps go back just the two of you and have a fun swim?

Sounds like she did get a fright when she jumped off. One idea might be to play around that angle... say you're in the water and she can jump into your arms. First try catching her without letting her head go under, then lead to having her jump in on her own and you grabbing her after she lands etc?