The use of 6090 kHz and other frequencies within the 49 meter broadcast band during the daytime almost points to them using NVIS as their propagation mode of choice...that, or they're even closer to each other than NVIS range. Hard to say for sure. Since they're certainly pescadores in the literal sense, and they operate out of the legal marine band (some of the time) they probably belong in the peskies forum.
The owners/captains of these boats likely had their radios programmed with several different frequencies that can be picked "on the fly" (channelized or programmed into the radio's memory) depending on time of day, interference and who they want (or don't want) listening in on their conversation. While 6212 kHz is one of the ITU standard HF marine channels, the other frequencies they've been heard on are out of band (either in 49 meters or in the fixed/mobile allocation that is 43 meters / 6765-7000 kHz).
Considering the sheer number of Spanish and Portuguese language chatter I've heard on the higher frequencies - that is, from roughly 7300 kHz or 7400 kHz up through 8 MHz into the bottom and middle portion of 9 MHz, I'm surprised these guys aren't using those frequencies as well. If they are within NVIS range of each other, they probably have a lower frequency (I imagine somewhere in the 3-5 MHz range) for use when 6 MHz isn't playing nicely.