Below is a post from another boating forum concerning the question of installing flow flow gauges.
"I guess I'm going to be the voice of decent on this one...........
With 3126's you will need to plumb senders into both the*fuel supply and return lines which means the cost for installation is going to be close to 2X what it is on a gas boat. Every place you break into a diesel line is one more place for a leak.*
Aside from the cost, I also question the need on a Cat powered boat. I use Caterpillar*fuel tables and find them to be right on the money as far as GPH. On a 300 gal fill up, I am usually within 5-10 gal of what I expect and have computed from the tables.*
Using a*fuel flow meter to spot something wrong........? On a diesel if you are observant, you will hear or see problems quickly by smoke, smoke color, sooting, starting problems, acceleration problems, etc. many times before you see it in*fuel flow.
As far as using*fuel flow to tweak running attitude, keep your eye on the ball here: set the rpms at the*fuel burn you are comfortable with (from the Cat tables), then trim the bow up or down until you achieve the maximum speed at that rpm setting. That is a lot easier than investing $1200 and 3-4 days installing*fuel flow meters and it gets you the same result."
My take: if you've got the money why not? However with a little effort the same info can be computed without spending the money and you will know your boat and engines much better.