I agree with the others that you have to do some research before you book a cheap AI hotel as some of them can be dreadful.
One you could have a look at that is ok for a cheap AI is the Eken Resort Hotel in Gumbet. I'm not sure about water slides but it does have a decent pool and is near the beach and close to the nightlife of Gumbet. There is a very good water park you can go on a trip to as well.
I don't know when you are planning on going but if you plan to go high season you may not get much for £400 AI, bearing in mind that flights cost approx. £300 alone, that doesn't leave much for accomodation, food and drink, especially if you are talking 2 weeks. You really do get what you pay for with AI hotels and if it is really appaling you will have to go out and eat and that will put up the price of your holiday.
I hope you find something. Coma back and let us know the hotel you chose before you book and we can let you know if it is a decent place or not.