temp sender reading too low

The temp of my engine does not seem to rise above 125F even though I have a 160F thermostat.

To date I've changed the stat twice, metered the wiring between the gauge and sender, checked voltage to the sender, replaced the sender and gauge and I still get the same problem. The boat does have a heater take off on the manifold so the stat is not in direct contact with the block any more. See link photo.
http://www.pinkbike.com/photo/2001387/

I can't for the life of me figure out what the problem is, I've used a laser thermometer and get a block temp of around 140F near the manifold and 150F on the block just beneath the cylinder heads. My question is, could the sender in it's new position account for the 30 - 40F drop in reading or do I have some other cooling problem with the engine??

Thanks for the help.

Interesting that this same problem just came up on PN. I as well as another feel that moving the sender away from the block is the problem. The easy spot for the heater supply is a block drain but this water may not be as hot as the water at the intake manifold. Some manifolds may have two ports so I'd look at that too so the sender doesn't need to be moved.

Here's the thread: adding a heater thread on PN