"How do I get there? Do I have to take a ferry?"
Yes! You get to take a ferry. Or possibly a float plane :) See the schedules below.
Note that while you have to pay a ferry fare at Horseshoe Bay, you don't pay at the Langdale terminal. The fare is automatically round-trip.
This will get you to & from Horseshoe Bay by public transit. These buses don't go onto the ferry, so you get to be a walk-on passenger if you travel this way.
This is public transit on the Sunshine Coast, which you'll need if you're a walk-on passenger on the ferry. You'll be wanting Route 1 (Sechelt), starting right at the ferry terminal and going all the way to the end of the line at Trail Bay Mall in Sechelt, then Route 4 (Halfmoon Bay) to Coopers Green Park. The park's not a time-listed stop, but it's between stops P & Q, in that order. If you need clarification, it's best to call the phone number on the website.
Harbour Air runs round-trip service from Richmond, downtown Vancouver and Nanaimo to Sechelt.