Listing some considerations:
- political risk / proximity to potential threats
- cost of shipping goods inbound
- cost of shipping exports to markets
- weather events and other natural hazards
- availability of energy resources
- availability of mineral resources
- availability of fisheries resources
- availability of marine nutrients in surrounding waters
- ease of access for tourists and short-term residents
What have I missed?
Other thoughts:
Cost of shipping doesn't require absolutely close proximity to the land nations concerned as long as the seastead is close to a shipping lane.
If the seastead can move, even very slowly, then many hazards can be circumvented / defused.

