Isla San Martín sits in the middle of the river between the major Argentinian and Brazilian cataracts, a small rocky outcrop of about half a hectare that you can only reach by free shuttle boat from the Lower Circuit (when river levels permit). A short, steep climb takes you to viewpoints that are, in some ways, the best the park offers — you are surrounded on three sides by falling water with no other tourists in the frame.
The boat does not run when the river is high (typically December to February), and even in dry season the schedule is weather-dependent. Check at the park entrance in the morning.