but the admin center itself has to be on land
I took a glance at the code, and it doesn't seem like that should be the case. Does the settler unit have can_swim set to true, and the admin center have the right placement restrictions?
<construction seasons="0.2">
<placement_restriction type="not_walkable"/>
<placement_restriction type="swimmable"/>
<placement_restriction type="no_structure"/>
</construction>
These restrictions should result in a structure that can only be placed on deep water.