There is a phrase used in English for what I think you're describing, but it's a verb phrase rather than a noun. Touch bottom is used when a swimmer is in water shallow enough that their feet can reach solid ground while their head is still above the water. Here's a dictionary from Oxford Dictionaries, with some relevant examples:
Reach the bottom of a body of water with one's feet or a pole.
‘My feet touch bottom… It's cold in the water, but I'm warm.’
. . .
‘He finally struggled close enough to shore so his feet could touch bottom, then he just stood there with the water lapping at his neck.’
. . .
‘Her feet touched bottom and she stood up slowly, revelling in the water flowing from her as she rose from the pool.’
If you wanted to use this phrase in your sentence, you could say something like
He had to make a couple of fast strokes so his feet could touch bottom, to just feel sort of safe from a possible shark attack.
You could probably just say "so he could touch bottom", but using "his feet" clarifies that you don't mean that he swam down to the ocean floor (to be below the sharks, maybe).
Of course, to be actually safe from shark attack, your hero should really get all the way out of the water (sharks can bite in very shallow water), which (from the ocean) means on the shore or beach.
Two notes: It is possible to say that someone "touched bottom" when they swim down or sink to the bottom of a body of water that is over their head, but this should be clear from context. Also, there's a different, figurative use of the same phrase to mean "go as low as you can" (it's definition 1.2 in the same ODO entry; more often phrased as "hit bottom" in the US). Again, this meaning should be clear from context.