I have a 2 year old 1860 Tracker with a 90 HP 4 stroke Mercury engine.  I believe there is an in-line fuel filter located inside the engine hood but I was wondering if it would be worth the expense to add an external filter adapter and a spin on filter at the rear near the engine in an accessible location?  I run non-ethanol fuel and have not had any fuel related problems yet.

Probably not worth the effort/ expense......on the other hand that's a nice boat and a pre-filter sure won't hurt anything.
I just added a cheap " universal" filter to my old Mercury.It has the advantage of being clear so you can actually see the fuel.

I don't see a lot of external filters installed unless maybe they are fitted to a carry on type fuel tank.  Then they are usually a clear lawn mower type filter.

Yes,my current boats both have carry on tanks.....a real
PITA.

If it's the 2.1 liter 90 hp EFI unit Merc does not recommend an external fuel filter/water separator unless it's

absolutely necessary. Those motors use a "fuel supply module" which does not like any restriction

between the tank and the fuel inlet hose on the motor.

We have several in our rental fleet and they are very sensitive to restrictions. I recommend changing the

in line fuel filter under the cowling regularly. Like 2 or 3 times a season if you use it a lot. It's easy to change

and takes less than 30 seconds once you've figured out the quick connectors.

I checked the info tag under the cowling.  It was listed as 2.061 L. so I suppose it is a 2.1.  The manual indicated the low pressure filter which is replaceable by routine maintenance and the high pressure filter that only should be serviced by Techs.  All sounds like good advise, so I will just leave well enough alone and stick to servicing the LP filter as stated above.

There really is no need for an additional filter. In fact the clear inline ones often cause more trouble than they prevent. The newer fuel systems do not like the extra restriction. They have a poor record of failing and leaking fuel into the hull.