It's soon after (going north) the 7 Mile Bridge at MM 48.5 bayside. The first tour was 10am. We hadn't booked and the 10am was full (about 15 or 20 people per tour). We got the last 2 places on the 11am.
Important: also note that they only allow guided tours and there are a limited number of tours available. So on busy days (including rainy days) it can often get booked up. I REALLY don't want you driving over an hour out to Marathon only to get there and find out all the tours are booked up for the entire day. And it happens (i.e. it's happened to me once). If you're not on a guided tour there is nothing to see at the Hospital.
Then we go outside and to the tanks to see the residents. There were 20 turtles to see.
A great place to visit - very recommended.