Upvote:3
Planning an unplanned trip, eh?
But seriously, there is only Singapore, Malaysia and Vietnam. The short way to Malaysia would be over land, towards Kuching. One of the larger ship operators doesn't go directly abroad from Borneo, but you can go to Jakarta, then Sumatra and go on from there.
Be aware that the ferries there are in terrible conditions, often have accidents and the distances are up to 1500km. A 30 minute flight might be preferable on many levels to 4-5 days on these ships.