J.B.
One of my brothers has had a seizure disorder since he was 3. It never interfered with his social life. My mom would just tell the parent about it, what to look for, what to do if they thought he was having a seizure, etc. It was no big deal at all. Lots of kids have health issues that require that the hosting parent be extra careful with - food allergies, fear of pets, diabetes, etc. Communicating your concerns and any steps they need to take, just like you did with the sleepover, is key.
Regarding pools...again, communication is key. I know there are parents out there who let their kids swim with minimal or no direct supervision. I am not one of them. When my kids are in the pool, I am sitting on the pool deck or in the water. I think that if you were to explain your concern for extra vigilance around water because of the risk of drowning while having a seizure, any reasonable parent would assure you that of course he or she is right out there with eyes on the kids all the time. Those that waffle a bit, or say "oh they're fine they'll yell out of they need anything" are the ones to avoid.