Since the lightning would go for the heighest conductive point on the widest surface, probably staying inside of the water...

At least, in the low probably of being hit by a bolt, the water could disperse the charge faster than on the land, hence giving you more chances of surviving...:-)

The beach is safer since the lightning would strike the sea, which is much more conductive. The slightly greater height of the beach is not significant conpared with the height the lightning has already travelled. The current only flows through the surface of the sea, so you would also be safe if you could hold your breath and go underwater for the duration of the strike.

You would be in trouble either way, once the sand got wet anyway. Lightning is fiendishly unpredictable. Also it doesn't have to hit you to hurt you or kill you. A lightning stroke on wet ground has been known to kill entire herds of llamas lying flat on the ground. What killed them was the electricity flowing away from the point of the stroke impact, not the stroke itself hitting them.

Similarly in South Africa, a couple of years ago lightning struck a pole at the edge of a soccer field. Most of the players were knocked down and several had to be hospitalised, though, to my memory, none were killed. Those that hardly suffered probably were the ones that had one foot in the air when the lightning struck. Those worst affected had both feet on the ground, one further away from the stroke than the other. That would cause there to be a potential difference between the two feet. I suspect that more people get killed that way than by a direct hit.

Seawater is a fairly good conductor. I would not like to be in the sea if a bolt of lightning were to strike a wave within 200 m or so. But if the lightning were to hit the beach within 100 m or so of where I was defying the bolts in the water, I would expect to die. If I were on a wet beach I probably could survive anything but a direct hit more than say 10 m away, as long as I did not have both feet on the ground.

But as I said, lightning is largely a law unto itself. Don't ask me to underwrite you anyway, unless you head for the barroom, dry and safe, at the first hint of lightning.Of course, though I love both lightning and thunder, I am prejudiced because parts of the Transvaal where I used to live were on the shortlist worldwide for their density of lightning strikes. Even though it cannot rival the Witwatersrand, I don't know how Britain stacks up, but I do know that there have been fatalities even in central London.

This is adding some remarks/opinions to the hallowed Jon Richfield answer.

See this link for lightning strike density in South Afica

http://www.wattronics.co.za/wp/2009/07/28/lightning-flash-density-south-africa/

Notably the area around Underberg has 12 strikes/sq km/year compared to Witswatersrand's 6.

WRT wet sand on the beach, this would normally be by rainfall (fresh water - ignoring the possible "salt" water table) and in addittion to the higher resistance of the sand particles this would contribute to a higher step voltage than would be experienced in the sea.

WRT the height of a body above the ground level "attracting" a strike --

There is a "cone of attraction" extending downwards from the top at only 33-45 degrees that has a "significant" "attractive effect. In other words, the circle described on the ground by this cone would have to have been hit by the lightning were the attractant not there, for the attractant to be hit, were it there.

Having considered this much (due to many years of scuba diving, glider flying - in lightning conditions - and the repair/replacement of lightning damaged industrial electronic gear) I would prefer to be in the (salt) water or more preferably under it during a close storm!

Most lightning fatalities 200+ pa in South Africa (compared to 10 pa from snake bites, and 1/2 pa from confirmed spider bites) are due to step voltage and not direct hits. There are some remarkable stories of herdsmen surviving amidst many dead cows/sheep (because the animals have a longer "step" contributing to a greater voltage and resultant current - other factor may be due to rubber soles). Also this occurs with groups of hikers where the survivors were probably in mid step. Horse riders survive although their steeds drops dead.

Some interesting although crude and perhaps outdated figures:-

it require approx 14mA directly "across the heart" to stop it.

but only 0.3 mA from inside to outside the heart. (hence the much stricter grounding/earthing criteria required in operating theatres

Food for Thor(t).

How interesting Jon and Ecstatist! I had no idea about that! I would have tried to lie flat n the beach in order not to be a high point. The information you give suggests quite a different behavior, so you yourself don't mention it and I've never read or heard it: Forget the "don't panic" advices! Panic means instinct - knowledge how to save yourself that isn't in your head but in your genes! Run! Run as long as you can even if there is nowhere to run to! As long as you are in the air you are not earthed, and while a foot touches the ground you are at least a little bit bent, so less tall, and attract a direct hit less than when standing or walking upright, and an indirect one is not as dangerous than when two feet ore more points touch the ground.