I'm planning on taking a water taxi over to Klein Bonaire but I keep reading about people having guides. Is this really necessary? What is the purpose? I have never snorkeled with a guide? HELP!

I snorkeled Klein with my wife and children of 16, 15, 11 and 8 this summer.
It was wonderful.
From the "huts" at the beach, walk to the left for a couple of 100 meters and enter the water.
Booties are very convenient during this walk.
Just use your brains finding an opening in the corals.
Bring water and some snacks.

We encountered some small stinging jellies, so you might want to take measures.
I felt the stings, but they did not bother me very much; one of my daughters started screaming.

We saw many turtles and larger fish then we saw at the shore.

Depending on the number of people in your party, you might consider renting a boat. ($ 140 + gas)

BTW my best snorkel was from La Danai's leap to Karpata.

I've snorkeled to so many places, I rented the water taxi for $15 pp this past March.
We were the only two people on Klein. They drop you off and bring water and snacks, leave them
under the hut, and follow Bob's advice.
We didn't bring booties to get into the water because we just walked in. There are plenty of sandy places, and then drift around there.
We had currents when we went, but we just kept checking where we were. There are buoys in the water with the names of the dive/snorkel sites so we checked the Bonaire map to figure where and how far we wanted to go. Then we got out. Walked back north on the beach, jumped in and did it again.

When the water taxi came back about 4 hours later with another couple, we hitched a ride back even though it was earlier than we had told the taxi folks. It had gotten very blustery, and we saw just as good snorkeling, actually better off Bonaire! But that was in March. I don't think there is a bad place there to snorkel at all!! We loved it.