So looking forward to this game!
Many thanks for your kind words!
Do the player have any options on how to handle supply for the army when on campaign? Or navy?
Short answer is yes, the longer answer is yes but the naval supply will need to get some modest revision to match what is already programmed for land movement.
You can gather supplies in the field using the Operations Command menu of a city or a fortress (or a defensive wall):

- Assign Supplies Option.jpg (49.05 KiB) Viewed 1180 times
You can also make a decision to order the delivery of extra supplies - on a scheduled basis - at this location with the second item that you see. Usually, the decision to order supply deliveries will be governed by choices in the Administrative Interface. There is often (deliberately) some overlap between what can be ordered by a commander in the field and what can be done through Admin. And this is one of those cases.
Armies themselves also have a "Gather Supplies" item on their Ops Menu as well, of course.
Armies can carry those extra supplies with them when they move into a new area, or just keep them on hand for winter, etc. When moving, the supplies must be used first - before foraging - and a minimum of 1/2 of them are always consumed when moving. This is a simple, if abstract, way of showing the difficulty of transporting supplies by land in ancient times.
Those supply units must also be used first for the supply checks for armies at the end of a turn.
Now, there is also the option to build a Mooring if a city or a Fortress doesn't have any access to transport at all:

- Build Mooring Option.jpg (48.94 KiB) Viewed 1180 times
If there is at least road access, then you will see the first set of items. I will add a menu item to handle the situation where there isn't any water access (at an inland location) but there also isn't any land access due to a lack of a road. Probably it makes sense to allow a dirt road to be laid out quickly using an Ops Command in such a case. And it is pretty rare because most roads which can ever be built already exist at the start of the 69 A.D. scenario. But there are a few which don't. One example is Ireland (Hibernia). There is a road network which you can build but isn't in existence at the start of any of the scenarios.
Between attempting to gather supplies, when needed, and ordering deliveries to augment what is otherwise available, you should be able to manage supplies in key areas when campaigning there.
Note, however, that there isn't much you can do about water for field armies. It's too heavy to transport in large amounts (WAY too heavy). Arid areas will always be a problem in this regard. You do have an end-around in the sense that you can hoard your Extended Move cards for such a case. Water, when moving, is calculated as an expense of movement points. If you use an Extended Move card, that gives an active leader an extra 12 points and can reduce or eliminate the net effects of a lower water rating in a province.
Naval supplies can also be gathered using an Ops Command:

- Naval Menu Options.jpg (54.54 KiB) Viewed 1180 times
I haven't coded the effect of these cached naval supplies, however. I plan to do that when the land supply is more polished. Probably they will take the form of a new type of "supply ship" with rules somewhat similar to Supply Units used on land.