This is one of my favorite spots in San Marcos. Except when it's not. And that's when it's crowded.
The population of this little island is directly proportionate to the outside temperature on a given day. The day when I placed the cache? The high was 60 degrees Fahrenheit and it was very overcast; a lone fisherman was the only other person there. On a summer day in Texas, this place will be crawling with people: on the island, on the bridge, under the bridge, or even between the water and the land (with or without a rope swing). They'll be barbecuing and swimming, tubing and jumping, laughing and blaring loud music. And this island is primo property.
Summer holidays are even more crowded. Last summer, while shuttling tubers from bottom to top of river, I met a family who'd come up from Houston for the weekend; they arrived at the park at 5:30 AM to claim their spot. That's at least a three-hour drive!
Finding the cache in the "off season" should be relatively easy. But for an extra challenge, take on Muggle Island in the summer! Whenever you come, I recommend preparing to get wet. Falling in the river is a likely outcome or you may want to swim/wade/tube/canoe to the cache location. Also, please be very careful when moving around on the island. Aside from people, there are TONS of gnarly roots to trip on that get VERY slippery when they or your shoes are wet. That's the reason for the higher terrain rating.
Happy hunting. God bless!