There is "normally" a boat at 4:30 and 5:30 but I'm not sure about that schedule in the low season.
You can look here for more info: http://www.yelapa.info/
or maybe Robert, who lives in Yelapa, will pop in here and give you some first hand info.
The boats leave from Los Muertos pier at those times. Any airport taxi will know where the pier is. The taxi ride to the pier is about $235 pesos (for up to 3 people). The boat ride is around $100 pesos each person, each way.
Even if there is a 5:30 boat, you're going to be cutting it very, very close, if you make it at all.