Marine Aquarium Newbie

Hi All,

I have recently purchased my first marine tank, 63 gallons give or take, and
it is about 10 days into its cycle. I live very close to the ocean so it is
relatively easy for me to hunt around and pick up interesting little
creatures along the water line and in rock pools at low tide.

I asked the chap at the store where I bought my setup and he assured me that
I could happily collect things like hermit crabs and live rock (from deeper
water). He also said that I should have no problem using some sea sand I
collect there as a base in the tank.

Is this a safe thing to do? I initially thought that this would introduce
all manor of pathogens, parasites, etc.

If it is safe are there any precautions I should take? I've heard that
starfish can be quite destructive.

Thanks in advance.