She understands but is disappointed. This mermaid loves to collect things and Jack's hat was to be her favorite for a while. The others still want to eat Jack. But for now the negotiations are amusing them. The blonde, for her part, plans to take the hat when Jack is eaten. But for now she'll trade for something else.
She grabs hold of the side of the boat, pulling herself up to get a closer look at everything else. This might take some time. She'll have to investigate each thing for it's worth and going through Jack's possessions is half the fun. The other two are in a bad mood after their fight and hungry but the blonde is full. She doesn't see this lone man as a threat and is younger then the others. She has less experience with being fired upon or having things thrown at her. No one's ever tried to catch her in a net to bring her up to satisfy the crew. She pulls herself inside the boat, letting her tail hang over the edge. The other two bark warnings but aren't very insistent on it. Jack is alone after all.
Once the more social one is laying at the bottom of Jack's boat her tail swishes above the water. She studies the bit of map but doesn't want it. No pretty pictures and it wouldn't last under the water. She knows this. Next is the sack. This thing is looked at for a long time as she tries to discern it's purpose. It's turned this way and that. folded inside out... after several minutes she covers the end of her fin with it and leans back to admire herself, flapping it back and forth to see it at every angle.
She communicates with the other two, asking their opinion of how it looks on her. It's an inquiring purring noise deep in her throat, water held there to make the wet bubbling accent they speak in.