For both item and block tags:

• create:seats should include interiors:kelp_seat;
• interiors:chairs should include interiors:kelp_chair;
• interiors:floor_chairs should include interiors:kelp_floor_chair.

Currently, none of the above is the case, which is a small annoyance (for example, when grouping items with REI).

The reason kelp seats aren't included is for crafting—you shouldn't be able to craft regular wool seats/chairs from kelp chairs, or vice versa.

What rdh said 😁