Sure. You could take a cruise, take another flight. Heck, you could swim it for all AA or other OW airlines care! All a surface segment means is that you are not flying between the two cities involved as part of the ticket. How you from A to B is your problem, not theirs. Surface segments are a good way to avoid wasting a segment when the distance between the two cities is small and/or the price is cheap.

For example, on my current AONE5, I had an LHR-FRA segment, that I did not use as a surface segment. It made sense at the time as I was trying to not spend any more money than I had to and I needed to travel between London and Frankfurt. Had I done that as a surface segment, I would have been able, later in the trip to fly from Singapore to Melbourne via Hong kong, which would have been more miles and another opportunity to fly CX longhaul. Instead, I am "stuck" flying BA between SIN and MEL.