I have an 1972 wards walk thru boat i use for small lakes and creeks in my area. i have noticed that surface of the deck is getting small surface cracks. what is the best thing to use for a sealer to stop the spreadig of the cracking?

Are you are sure it is fiberglass and not gel coat getting stress cracks?

There's is nothing you can do to stop it.  Most likely what you are seeing is just strees cracks and they are going to show in places on almost any fiberglass boat after they get a little age on them.