Thank you for your query.
I do have an Alexander born 8 Nov 1817 in Tulliallan Perth. His parents were James WATT and Elizabeth WYND who were married in Tulliallan. Their son Captain James WATT was a Mariner and it is quite possible that Alexander could have been a mariner also.
I have no knowledge at this time if James WATT senior was also connected with the sea.
The main two trades in Tulliallan were mining and fishing.
At this point in time I have no knowledge of Alexander marrying at least not in Tulliallan.
I hope this helps you further in your research, if this is your Alexander it would certainly add to mine.