Bent the threaded tab on one today and will change it out since the gauge is erratic now. Why is the boat set up this way though. I only have 1 gauge but a sending unit in the thermostat housing and another in the intake manifold. Kind of like a pre and post thermostat. Merc part numbers show to be a 48952 in the manifold and 806490t in the T housing. Any ideas why it's set up this way?

Temp sender and alarm sending unit. Google was my friend

Yep. Alarm system is separate from gauges and use different senders. Tan with blue stripe wire is alarm stuff.