I`d always run a ring terminal around the gauge to a ground.

+1 above

If the sender has just one wire, you need to ground the sender since the spot you have your sender (in line) is not grounded. Don’t worry about ‘too much’ sealer in your case.

Simply run a ground wire to it from block ground point. Use a male spade connector and put flat against a flat of sender and use a hose clamp to keep it pressed against it there. I do this with 1 wire coolant temp senders on hard anodized intake manifolds because they are ‘electrically dead’ when hard anodized.

I may have other problems with this set up. The one wire post on both senders spins. I don't know if this is really a problem or not? Also, I touched the sender and ground in several locations and the ohm meter did not "tone" but I touched the post and ground and it did "tone". It doesn't sound correct to me but I don't know how it works. I will have to try one of the grounding techniques shown here to see if that helps. I may have broken them and will have to buy a couple new ones.

This really does not mean much. What was the actual resistance ?

Fair enough. I assume that this is how they work. I won't check till this weekend. Do you have any idea what the resistance should be? Do you think it's a problem that the post spins? Thanks for your help.
Do you think the problem is a "lack of ground"?

I dont know what the resistance is on that particular sender. Is there a part number on the sender ? You should be able to hook one lead of your ohmmeter to the body and one to the post and it should change as you heat the sender up. A hair dryer should do the trick.

I have never seen a sender where the post spins unless the post screws in and it has come loose.

Thats what I think combined with maybe the loose center post on the gauge. You should be able to easily check that by rigging a ground wire between the body of the sender and a known good engine ground.

Never had a loose post. That can’t be good.

As a bunch of us said…the body of the sender must be grounded.

I grounded the sending unit by zip tying the wire to it and the neg battery post and what do you know, it works!! It was a half hearted attempt because I really thought I broke it when the post started spinning. I only did it on the port engine. Anyway, I took it out and used my "Pedestrian" data logger (GoPro) and videoed several different runs. The oil was running at and slightly above the 230 degree mark when running hard. Might have been climbing but I ran out of river and had to go back to "no wake" It's 90 degrees here and probably not the best time for a WOT run to see what it will do but with the B-1's 26 pitch it looked like 5200 rpm and about the same 75 mph. The RH prop I think is untouched but the left has been worked on so not a matched set. I also think that the LH rpm was lower. I will try and post some pics with the info

I looked at the video and my data logger (GoPro) can't read the display either!!! I have ordered a shield that I hope will block the sun enough for me to read the gauge.

I took the boat out with family last weekend and the port engine is running hot according to the oil temp gauge the starboard was fine. No high speed runs just 3500 and below. At idle the oil would cool down below the 210 on the gauge and then go up to about 230 or so when underway. In an effort to locate the problem, I decided today to swap the sending units and now both are running hot 230-240 ish. Kind of a guess because the gauge goes from 210 to 230 then all the way to 320. I don't know if they will go higher or not, I have to slow down for the MANY no wake zones on the Grand River. Water temp never exceeds 160 and will drop as I run up the rpm. When I come off plane to idle, the oil pressure stops at 20-25 lbs. The engines run fine and oil pressure is great underway. I felt the lines (both engines) from the thermostat and the one to the cooler is hot and the return is cooler as expected however the line that comes from the thermostat back to the engine is hot. It's inline with the return from the cooler. How is that possible? The coolers are the 3"x23" good for way more hp than I have. Everything is new. Is there something else I should be looking at before I pull the boat and put it back in the shop? Thanks for your help.