Well, the short answer to your question about what to see is "not much".
There are only 4 types of life I have seen. Clams, trout, crawdads, and sculpins (picture attached). I just moved up here so time underwater at Lake Crescent is not much but talking to other divers I have seen it all.
Where is either East Beach or the North park seavonj mentioned. Both have about 50 yards of shallow water to wade out in before the drop off. Basic facilities are available at both.
Lake Crescent is usually the alternate site if the Straight is unsatisfactory. If you are coming up here I would highly recommend stopping in and seeing Mike at Scuba Supplies. He has been in the area for years and will give you a good list of sites to try.