@beelee Should be possible. Try placing a construction with isSea attributes and also give it production attributes. But confirm that your current settings allow for construction placement without factory otherwise will need to build next to a factory.
Heres a few to check and or use. refer to POS2.xml for complete lists
canProduceUnits
values: allows a unit to be a factory, without all the other things that come with being a factory (like capturable, land, non-combat, etc). So if you want flying factories, go ahead. Just don't complain if you get errors.
isConstruction
values: allows the unit to be placed in territories that don't contain factories.
Only "constructionsPerTerrPerTypePerTurn" contructions of "constructionType" may be placed per territory per turn.
May have only "maxConstructionsPerTypePerTerr" constructions Total per territory with/without a factory unless you enable "More Constructions with/without Factory", which will allow up a total number of up to the territory value.
"Unlimited Constructions" game property allows unlimited constructions total in a territory.
constructionType
values: is just a unique string which identifies what kind of construction this, if two or more constructions have the exact same constructionType (and the same PerType rules), then they will follow the same rules for placement.
example: if you have little_bunker and big_bunker both as type "bunker", with constructionsPerTerrPerTypePerTurn=2, then you could place 2 littles, or 2 bigs, or 1 little and 1 big in a territory
If you do not want this unit to be effected by the global properties "More Constructions with/without Factory" and "Unlimited Constructions", then make sure the type ends in "structure".
example: "harbour_structure"
constructionsPerTerrPerTypePerTurn
values: is how many of this type you can place in a territory every turn
maxConstructionsPerTypePerTerr
values: is how many max of this type may be in a territory. this must be greater than constructionsPerTerrPerTypePerTurn
if "More Constructions with/without Factory" = true, then the minimum will be either maxConstructionsPerTypePerTerr or the PU value of the territory (whichever number is greater) in territories with/without factories
if "Unlimited Constructions" = true, then this number becomes 10000
Also @wc_sumpton is correct the sezone may need production values.