You don't necessarily have to use water to land, unless your desired landing weight necessitates the extra performance/thrust that it provides. If water is required then it can become a limitation since you only have 90 seconds of it on board. Of course water doesn't begin to flow immediately in the decel and you may be light enough to hover without it by the time it's exhausted, so that doesn't mean you can only hover for 90 seconds. Ultimately NATOPS limits you to 5 minutes of continuous hover followed by an equal cooling period.