I have recently had Botox (3weeks ago) and have today been snorkeling and now have a mask imprint along my Forehead?

No worries as this is not a result of your Botox treatment. It will resolve. I too love to snorkel on vacation and have seen this on my own forehead for several hours. :)

The prominent marks on your forehead are likely due soley to the pressure exerted by snorkel mask, and not the Botox. I suspect the imprint will fade over the course of the day.

I also agree this isn’t a cause of Botox and simply a mask imprint that should resolve within a few hours

Anyone who wears that mask for a few hours is going to have a prominent mark and it doesn't go away immediately. This is unrelated to the Botox. It should resolve gradually over the next 24 hours.

The impression from the snorkel mask is unrelated to the botox, it should get better several hours after removing the mask.

The imprint has nothing to do with the Botox and everything to do with your mask being too tight. It will resolve, but it has nothing to do with your Botox treatment. "This answer has been solicited without seeing this patient and cannot
be held as true medical advice, but only opinion. Seek in-person
treatment with a trained medical professional for appropriate care."