As a non-scuba-diving lifelong snorkeller, I won't presume to pronounce on the suitability or otherwise of Speedo Nemesis fins for whatever underwater activity you are planning to do.
This said, the idea that all fins marketed as suitable for swim training are by definition unsuitable for any other aquatic pursuit isn't accepted by everyone. Things aren't always so clear-cut and specialised in practice. It can also depend on the ability and expectations of the individual. I snorkel in the sea using traditional full-foot fins that many "modernist" divers would dismiss as swim workout fins.
I'm not alone in preferring old-school fins to today's plastic concoctions marketed as "snorkelling fins". I love the article "My favourite kit" at
www.divernet.com/gear-features/p300505-my-favourite-kit-pete-atkinson.html
Here's what Pete Atkinson, an Australian professional underwater photographer, has to say about fins:
FINS
I currently use the best fins I have ever used, but no dive store in Cairns will stock them, as they aren't profitable enough. They are orange and blue, Malaysian rubber fins by Eyeline, available from a local sports shop for £20.
From new, I could snorkel for a couple of hours without any hint of blisters. They are stiff enough that I can push a Seacam housing around all day.
For the diving I do, such full-foot fins are by far the best. Manufacturers continue to dream up fancy expensive gimmicks to extract more money from us. I'll concede that a few of these might actually be useful but, offhand, I can't think of any.
And here he is with his favourite fins:
I've got a pair of Malaysian Eyeline fins similar to his, priced £16.80 (roughly US$25) and they not only do the job, they're very comfortable too.
And like him, I bought them online from a general sporting goods dealer, as no British dive store, physical or online, seems to stock them!
In conclusion, I must re-emphasise that I have no brief for Speedo Nemesis fins. To be honest, I wouldn't use them for my own snorkelling forays because they lack the classic simplicity I look for in fins. My message is just that there are some fins versatile enough to be used in multiple aquatic pursuits.