Did two dives there two years ago in November. Water temp 34 degrees air temp 28... If they offer hot chocolate take it.
The dives themselves were really cool. Most of the dive is really shallow unless you drop down to the caves at 140ft right at the start of the dive... Don't do that though. Water is super clear and drinkable, so if you get thirsty on your dive, take a swig.
Enjoy and make sure you check out the rest of the park while you are there. If you can swing a glacier hike in also I would highly recommend that. Try the puffin, try the whale and try the lamb hot dogs. Brennivin is evil and don't even take a taste of opal.
The English pub was my favorite place to hang out, they have the wheel of drinks which If you are lucky can save you alot of money.
Enjoy your trip. Eg er Ekki fatur. (that's the only thing I remember how to write in Icelandic.)