Hi Carl, there's loads on the forum about this, so make good use of the search facility. Try searching on keywords taxi & airport, water park, airport & transfer, hire cars.
I personally would get a hire car, then you're able to get out of the area and see more of Tenerife, but if you don't know the island well, then maybe a transfer from the airport to the hotel might be best in the first place.
If you get a taxi, I'd expect to pay around 20 Euros, though the cost will vary depending on time of day and taxi driver mood!
Good luck, and have a brilliant holiday. I've retitled your thread so that others who are looking at it will know immediately what it's about and it might generate more answers from people who know about the subject who might otherwise not look at it.