"When you finish a long rest, you can summon a different primal beast. The new beast appears in an unoccupied space within 5 feet of you, and you choose its stat block and appearance. If you already have a beast from this feature, it vanishes when the new beast appears."

So you can only summon it first thing in the morning? If you could do it anytime (once per day) then you could simply resummon when you reach the water. And hope you're not coming back out of the water the same day.