Put another way-if one person is swimming in a pool for an hour and another is lying on the deck, how much more of a tan will the deck person have than the swimmer?
I don’t know, but I think you’ll find that the water not only reflects some of the light, but also acts somewhat like a magnifying glass for some of the light. I’m not sure it’s a given that the dry person will have more of a tan/burn than the wet person.
Here’s a pretty good post on another site. In summary, it looks like about 8% of UV is reflected and about 1% is absorbed (at 1 m depth). So a swimmer gets about 90% of the UV that a sunbather does.
UV goes through water so well that quite a few fish see UV and have interesting coloration visible only in UV.
Note that it gets complicated further by the issue of how much the person in the water is receiving reflected light exposure to the points out of the water compared to the person on the deck. And that the reflection component is greater due to surface turbulence.
So being in the water is like SPF 1.1.
Note that that’s for light angles normal to the surface of the water (for the sun, this only happens if you live somewhat near the equator, and only at select days during the year), and with completely still, distilled water.
And at noon.
While fully submerged?
Posture is also relevant: If the person on the shore is lying down, and the person in the water is upright, that will make a significant difference right there (given that, in swimming weather, the Sun is probably closer to the zenith than to the horizon).
Yes… and for spherical people with no friction. 