You can't out run a deathsquito not even if you are unencumbered and have corpse run. You can't out sail them even in a long boat with max speed fair winds. As soon as it aggro'd you could have stood up and shot it with your bow. Deathsquitos hit hard but they have very little health. Even when they are aware of you one shot should kill them.

Another tip about boats. You can't deconstruct them with your hammer but you can break them with a weapon or your axe and get all the materials back. Stash all your inventory in your longboat so that you have both the room and the weight to pick up all the materials.
Make sure that you have auto gather turned on. Then stand in your Karve at the mast and whack it with your axe until it breaks. You'll drop into the water but all the materials for the boat will go right into your inventory before it gets a chance to sink.

You can do the same thing with your long boat. Sail to a new island, build a portal back to your main base or a portal hub with an open portal available to link the new portal and then take the axe to your boat and take the parts back home or you can leave it on the coast of the plains and just load your black metal scrap into the storage and sail it back home when you have a decent amount. Works the same way the other way around too. You can sail home with the ore and unload it then dismantle the boat, portal back to your plains area, and rebuild the boat.