It all depends on the route you choose to do a Corcovado Tour. The way from Carate to Sirena is partly done on the beach and another part is done in the forest, some rivers must be crossed and during the rainy season, they can get deep and with strong currents.
La Leona Station is very flat, and it is not necessary to bring rubber boots during the wet season, since the trail by the beach
It is easier to do Los Patos starting in Los Patos Station and finishing in Sirena Station since you come from a higher elevation to a lower elevation. Los Patos can get muddy during the wet season.
This should not be a problem for those that want to visit Sirena Station, since the boat service runs almost every day from Drake Bay, and from Puerto Jiménez.
Sirena Station is mostly flat, however, it can get very muddy during the wet season.
San Pedrillo Station is a mix of flat terrain with some trails going uphill and downhill, it can get muddy during the wet season.