James has a point, the WRSI are recommended.
RE Alan's Rocker rotating, I have one myself and if it is fitting correctly I can shake my head upside down with an open strap and it won't fall off/rotate.
I do notice the strap stretches slightly when wet. Another thing that was obvious whilst underneath (!) the sluice is that the washing machine was 'grabbing' the peak and trying to rotate the helmet. I am now re-assessing wearing it with the peak on bigger water.
I can't believe I volunteered for this...