Signs and symbols: The meaning should be clear

We took a day trip to a nearby coastal community on a gorgeous winter’s day and wandered along the foreshore of the inlet around which the town is based. At one end of a pontooned-off, very calm, swimming area, we saw this sign:

What does this sign mean?

My husband and I both tried to figure out what this sign meant — neither of us had ever seen one like it before in all our travels, and we came up with all sorts of ideas, none of which were correct.  The most likely explanation I came up with was ‘swim to the left of the sign’ (the marked out swimming area was to the left of the sign).

What do YOU think it means?

The first sign we passed didn’t have any text associated with it, but the one at the other end of the swimming area did. After reading the text, the sign was a more understandable, but I have to wonder at the purpose of a sign where the meaning isn’t obvious from the symbols.

Here’s the sign that was at the other end, with the text that the first sign didn’t have:

Beware strong undercurrents - what a strange group of symbols!