Territory Attachment, and Unit Attachment, will allow sea units to blockade territories. (squid daddy) (may need to set sea zone to have exactly 0 production for this to work, haven't tested yet)
blockadeZone values: true or false, can this territory be blockaded by neighboring sea zones?
<attatchment name="territoryAttatchment" attatchTo="27 Sea Zone" javaClass="games.strategy.triplea.attatchments.TerritoryAttachment" type="territory">
<option name="blockadeZone" value="true"/>
</attatchment>