Start taking tylenol now, if you can take it. It takes a sunburn 24 hours to get as bad as it is going to get and the tylenol stops it and also helps with the pain.
Next.. pure aloe gel is great, or as close to pure as you can find. Put it in the refrigerator and apply it cold liberally and don't try to rub in. Yes, it is sticky until it dries, but, you will get great temporary relief, then apply it again when you need to. Wear your bathing suit top if you can and your shirt wont' stick to you, or let it dry then put on a loose one.
A soft cold shower will give you some relief as well and stop the heating too. Don't use any lotions to hold in the heat.
Another that I have never tried but makes sense is to soak a washcloth or such in milk and apply to skin.
...here is a link with a ton of info
http://www.health911.com/remedies/rem_sunb.htm
If there are any blisters, not water blisters, call or get to your dr as soon as you can to be seen.
hope you get relief soon