Guess what?

Because I'm not sure. Maybe to create a safe swimming area so boats don't get too close? But the barrier must go down below the surface because the water is more still in this area.
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Update! @jgpittenger says, "They are pilings and log booms attached to block free floating logs carried by the tides from banging into the marina."

[Esther Rosenberg]

What a contrast in the water flow. Great shot with the reflections. Nice in black and white.

[Lou Ann]

Super interesting. You must be correct about the barrier below the water’s surface.

[Ricksnap]

Love the curved, diagonal and horizontal lines, reflections and ripples, great b&w

[Bill]

Like a photo with a mystery.

[gloria jones]

Nice composition to see the horizontal and vertical lines

[Pat Knowles]

Good to know the reason....I would never have guessed!

[Joan Robillard]

Fascinating

[Lynda McG]

Makes a great b&w

[haskar]

Great composition. Lovely repetition.

[Tracy Bousfield]

Nice shot.

[aikiuser (jenn)]

How interesting! Makes for a nice image

[Jane Pittenger]

I like the curve contrasting with the verticals and triangles. B and w is a good choice here

[eDorre]

Neat pattern!

[Taffy]

I have one (well, a lot) of these that I've never figured out how to process -- this came out really well.