ND Filters

Hi,

I was at the beach last month, and there was a storm out at sea.

I wanted to take some longer exposures.  But, even stopped all the way down, I couldn't shoot slower than like 1/8 or 1/15s.

I guess an ND filter would have been helpful (maybe that's exactly what they're for?).  I don't want to buy a bunch of expensive filters.  What do people usually carry/is there a particular filter, e.g. 1 stop, 2 stops, etc., which you find most typically useful?

Thanks,

-ira