That is very interesting. Many non subaqueous soils like inceptisols and entisols, are already without the diagnostic horizons. So there must be something else besides soil processes to justify the recognition of subaqueous soils. Of course there is the one process which would seem very much tied to subaqueousness, which would be limnic processes. Probably more a geological rather than a soil process. Thus limnic Histosols would seem to have their beginnings as Wassists if they are solely of marl, coprogenous earth, or diatoms.
I wonder if the deep end of the Cowardin wetland definition is of any guide as to the rationale for the group. As such, the original wetland concept would cover normal Aquic soils, whereas those that are too wet are lake and ocean bottoms. The definition allows wetlands to be defined as up to two meters in water depth, or deeper with emergent vegetation is otherwise present. The key criterion seems to rest on whether most emergent plants are able to germinate and take root, which ultimately rests on hydroperiod.
In my example, it would seem that the hydroperiod is not so long to exclude annual emergent plants, even if it might be once every ten years (don't really know). So I would be alright putting in the aqueous group on that rationale. Alternatively, the standard could require a hydroperiod long enough to exclude perennial emergent vegetation, since annuals could remain viable for decades in the substrate. In this scenario, the soil could be considered subaqueous. All this assumes that emergent plants require exposure or at least shallow ponding for germination. Deep germinating plants would have to start out morphologically as completely aquatic (thin tissues) and may have developmental constraints against forming enough structure to support itself above the water.
