Mainly for future consideration by the toad, feel free to pitch your own ideas. Not all of these are boat related, some applying to general inter-site interactions.
-When on an island isolated from any civilizations, make it so the parent civilization may or may not know about your fortress's existence, dependent on availability of hearsay or starting scenario.
-Allow expeditions to cross water, the amount they can cross controlled by the size and supplies available to the expedition. They will likely need a wagon to transport any boat components to shore, and a shore of appropriate roughness for the boat they're planning to deploy. You could also send an expedition out specifically to build a rudimentary dock to allow bigger boats. This would allow inland fortresses to potentially connect to the outer world whilst in isolation, though this would depend on the distance to the nearest coast. Things could get hopeless really quick when all you have is seven dwarves, many hundreds of miles away from a smooth highly savage coastline, in the midst of a Terrifying ocean, with mountains in the way...
-Setting up a coastal fortresses on both coastlines in question, with docks and boating infrastructure, and retiring them should help mitigate the above point.
-Allow expeditions of diplomats, issuing a general desired goal and the flexibility and resources they could agree on to achieve it, when discussing situations with other civilizations. Certain topics include:
--Asking about events occurring at the visited site or informing the site of specific or general events with a degree of authority above general rumoring.
--Request the general expectation of a trade channel going either way, encouraging sites to send you a caravan or to send you a "caravan wishlist" to increase the chance of giving them the items they desire.
--Declaring or requesting changes in the inter-site relations of the site in question, with a given chance to declare 'a priori' for such, or lack therof, or even lie about it. The latter two will likely cause your diplomat to be upset based on their ethics, will make other sites and civilizations trust your fortress less due to it, based on their ethics (including your home civ), and will cause unrest if your civilians catch wind of this action, based on their ethics (which means absolutely for default dwarven ethics). Anything up to and including demands at threat of war or war outright.
-Add "kill diplomat", "kill civilian", "kill surrendering", and "war no a priori", to ethics, to allow dwarves and other civilizations to react to such incidents. The first four should ONLY apply if the kill target is in any combat level at brawling and above.
--A unit voids their ethical protection in the first three mentioned ethics if:
---They initiate lethal or higher levels of combat.
---They go into No Quarter at any time during combat.
---They raise the combat level after successfully surrendering.
--They will still be affected by other kill ethics however. Dwarves can be given specific actions for combat ethical situations, and will attempt to adhere to them based on their ethics and discipline, or can be left to decide on their own.
Just a few ideas.