One of my favorite little private lakes has a "landing" on a well maintained, fine sandy beach which forces me to back the truck in far enough that the tail pipe is a few inches underwater.

Is the back-pressure this creates harmful enough on the engine that I shouldn't do it for a couple minutes here and there?

Should I leave the truck running or turn it off while the tail pipe is underwater?

I worry about it getting stuck after I back it into the lake. I got stuck in 4 Hi last time out on the beach, on about the 15th time I used the landing. >

Thanks.

It will create a bit of back pressure, but I doubt it will do any harm to the engine. 

On many inboards, the exhaust outlet is underwater unless the boat is moving forward.  I'm sure you've heard the characteristic, blub, blub, blub of inboards while idling at a dock, or moving in reverse.

Two cycle engines are a different matter, but even they can deal with the exhaust being underwater, whether they exhaust through the prop, or an exhaust which comes out behind the prop. 

It's not going to hurt anything if the tail pipe is under water. As for leaving it running, that's up to you. Personnaly, I never switch my vehicle off while loading or unloading my boat. I figure, while it's running, I don't have to worry about the grimlins doing some strange thing to keep it from starting at a worse possible time. As the old saying goes "Things Happens" and usually at the worse possible time.

Another thing, I would never use 4H in soft sand. 4H is only useful for slick, hard surfaces. There is a much greater chance to slip a tire and it doesn't take but a 1/2 turn for it dig a hole that will stick you. Anytime I'm in soft material, I use 4L and locked in second gear. Unless you have a massive amount of torque and hp, the engine will not have enough power to pull in 4H, and 4L, low gear will not give you enough tire speed if you start bogging down and need to pour the power to it to get through.

Hey, thanks guys, good to know. I'll try 4L in 2nd gear next time. I suppose 4H would be best for the slick, hard surface of a wet concrete boat landing.

NEVER TURN OFF YOUR TRUCK IF THE TAILPIPE IS UNDER WATER. 

I used to do this all the time when fording rivers and what not in Arizona. The problem you can run into is if you turn it off the water has a chance of getting back to your engine.  If you leave the truck on the pressure keeps water out of the pipe all together.  It definitely won't hurt anything unless while you're running you get some water into the air intake, in that case you're screwed.

Yes, keep the engine running, water can get in the motor and ruin it.

Happens quite often when doing this (wasn't me though)

With my boat and truck, that's how I know I've backed up enough to launch. When I hear the "glub glub glub " of the exhaust, I know I'm there.

NEVER TURN OFF YOUR TRUCK IF THE TAILPIPE IS UNDER WATER.

I used to do this all the time when fording rivers and what not in Arizona. The problem you can run into is if you turn it off the water has a chance of getting back to your engine. If you leave the truck on the pressure keeps water out of the pipe all together. It definitely won't hurt anything unless while you're running you get some water into the air intake, in that case you're screwed.

Water would have to flow up hill about 3 feet or more vertically for you to get water in your engine because a tailpipe was in the water when you were backed down a ramp. Its just not going to happen. The backpressure is of no real concern either because the gas can escape relatively easy.

I would leave it running though simply because you never know when bad luck will strike and your starter or some other related item goes.

If you have a "Reese Hitch"; for about $30 you can buy an extension that puts your trailer about a foot or so farther behind your truck. In my case that was enough to solve the tail pipe (and the wheel bearings and brakes) in the water. I think I bought mine on sale from Cabala's.

With my boat and truck, that's how I know I've backed up enough to launch. When I hear the "glub glub glub " of the exhaust, I know I'm there.

That might be ok if all ramps were the same pitch.  But, they are not.

A much better indicator is the fenders of the trailer.  If mine are even with the surface of the water, give or take an inch, then the boat can be driven onto the trailer.

It must be a bit deeper for the boat to float off the trailer.

With my boat and truck, that's how I know I've backed up enough to launch. When I hear the "glub glub glub " of the exhaust, I know I'm there.

That might be ok if all ramps were the same pitch. But, they are not.

A much better indicator is the fenders of the trailer. If mine are even with the surface of the water, give or take an inch, then the boat can be driven onto the trailer.

It must be a bit deeper for the boat to float off the trailer.

Good point. I rarely launch anywhere but two lakes and their ramps are the same incline. Your advice is well taken though.