I've been playing 2WIN as the Allies vs Elmer, getting ready to play Goliath in a continuation of our learning processes here.
I've noticed something that seems weird to me: if I order an Allied naval unit to provide *direct* arty support for a specific attack (say, 502 Para on one of the fixed CA batteries), *all* my naval units fire in *indirect* support of *all* my attacks.
If I give *none* of the naval units direct support orders, *none* of them fire in indirect support of *any* attack (according to the detailed combat results).
Since there's no equivalent of "D/E/F" or TR/LR deployment for the naval units, how are you _supposed_ to set up your naval units to provide indirect support?
This is a consequence of scenario designers using a different icon background color than the side's ground units use, and then assigning the naval formations a lower cooperation level. See the table on page 66 of the pdf manual. Anything less than Free Support for either the naval formation or the ground formation will result in at best limited cooperation between the two. If the designer has set the support levels at even more restrictive levels, as I believe Brett did with 2WIN, then there will be no cooperation between formations. However, there is a documented catch, or workaround, which you seem to have discovered accidentally. That is the cooperation levels for the battle will be determined by the highest level of cooperation available for the group.
So, any unit with a Free Support will end up boosting the cooperation level to free, among all the participants. Two or more units of the same formation, will end up getting the benefits of free cooperation if one of the formation is set to attack, and the others are pulled in as support units (assuming the comm check is passed). This is what is happening for your naval units. Since they have no cooperation with the land units, they generally need to be directly assigned to attack, in order to bombard in the attack. However, by assigning one, you have now invoked the free cooperation level of the remaining units, when they join in.
Some designers make this choice as a conscious design feature, while others may be unaware of the nuances of the cooperation aspect of the engine. Bob Cross (aka Curtis Lemay) uses this in his Okinawa scenario. I don't know if Brett was aware of it, or not. When I play the Allies in this scenario, I usually concentrate my naval units firepower on specific tough hexes, rather than take dilute them, and take chances with comm checks failing, half support values, etc. You have to be a little more careful in how far you move them in between attacks, if you choose this route.
Your mileage, of course, may vary...