Sea glass is simply glass from shards of glass which are weathered over many years of tumbling in the oceans. Dating of sea glass is very inexact. Much of it is based on the color and knowledge of when glass of such colors was commonly used. Collectors and people who make jewelry with sea glass have written extensively on which colors correspond to which dates.

Color alone may be insufficient though, and characteristics such as the thickness of the glass and prevalence of bubbles can aid in assigning an age to a specimen.

The following sites talk about more and less rare glass, usually based on their colors:

here
and here

You'd probably need to analyze the chemicals in the glass to get a better idea of how long ago the glass was made.

If the glass has been exposed to the sun's ultraviolet light, then you can track the amount of certain chemicals created by ultraviolet irradiation. I don't know what those are, exactly - you'd need to talk to a chemist.

It will be harder if the glass is far enough underwater to not be exposed to ultraviolet light (water blocks ultraviolet). I don't know if any types of glass use carbon that you could use for carbon dating.

You may be able to use properties such as color and amount of gas bubbles to find out what the original source of the glass was, for instance, soda bottle, a piece of someone's glasses, etc. Color can help identify potential processing steps of the original item and even the country of origin. In this case, knowing history helps because green was one of the common colors for medicine bottles. As to the amount of gas bubbles, older manufacturing technologies usually meant more bubbles or other kinds of irregularities in the glass. In some cases, size helps, too, because the thickness of glass objects that were made has decreased over the years. In any case, what you'll need to do is observe the glass and record as much detail as you can, and read up on types of bottles and other glass items that humans have used in history by location, and go from there.