There are two types of ticket on sale in the UK:
1) Ultimate tickets which are valid for 14 or 21 days unlimited admission from date of first use. Includes all parks (including water parks).
2) Premium tickets valid for 5 or 7 days admission to theme parks plus 5 admissions to the water parks. The 5 days at the 4 main parks plus the 5 additional visits, all expire 14 days after first use of the ticket.
However, there is little difference in the prices of the 5 and 7 day premium tickets and the 14 day ultimate, so it may be worth considering the Ultimate ticket as it gives you the flexibility to visit a park for the morning, just for the fireworks of an evening, or to combine a morning at a water park with an afternoon/evening at one of the theme parks.
Check the link given above and this will explain the tickets in full.