I agree with PP above about your options. My oldest DD wears glasses and my youngest son is actually severely visually impaired to the point where he is legally blind out of one eye and no prescription can fix his vision in that eye. He sees 20/400 with his glasses on.
With my older DD, we did 2 things. She is a swimmer, so she has prescription googles. Our experience with them is that they weren't that great. They fog up a lot (we have tried many tips to stop this) and of course, when you take them off, you can't see! So you need your glasses right there or you have to keep the googles on for the entire time. This past year, we got her contacts. It is much easier for her now and she wore those snorkeling in Cozumel and did great. But she is older and I do not know how old your girls are? Contacts are a big deal and a lot of money, so if you are just needing them for one day, it obviously isn't worth it. Also, I will say if their prescription is not that severe, than I would just skip it. They will be able to see some things without glasses and I would think they would still enjoy themselves. My DD swam without a prescription for 3 years before we invested the money into goggles. For reference, she has a -3 prescription in both eyes. She has never complained about this and has snorkeled with her dad in the past without glasses and done just fine. She still saw fish and had a blast.
My son, being that he is in a whole different boat, does what he can. We took him snorkeling as well and he did not wear glasses or prescription goggles. He doesn't swim that much and it just was not worth the price right now for us to invest in that. He is obviously too young for contacts. He still did it. We helped him and to this day he still says he loved it. I think he liked the boat the most though. If we are just swimming with him, he will wear his glasses as he doesn't like to put his head in the water anyway, but for snorkeling we took them off and had him wear just the ones they gave us.
I also wear contacts but as a kid I did not. I have a -8 prescription in both eyes, so can't see much of anything without my contacts/ glasses. As a child I swam all summer without anything and managed. We lived on an island off the Maine coast and I spent all day diving under the water and still had a blast. I think kids that wear glasses are kind of used to not always seeing great! I am going to bet your girls will have a blast no mater what you choose to do. Although I understand your concern, as I have that concern daily with my son as well. Good luck with your decision!