if you team up with 2 other fishers you can do it many times a day.
Now that it's in guide section already, here's how you get it.
you have to be able to recieve Inside the Belly quest first to get the trigger.
http://ffxi.killvoid.com/page.php?id=728
the pre-req for this quest in addition to 3 other oswald quests, are fishing skill 30+ and the sand charm quest in mhaura.
trade bhafhel marlin to zaldon, you may recieve brigand chart, or pirate chart. The chance is very random. I've got it 1/1, I've got it 1/200+. The pirate chart is the one you want.
Once you get the pirate chart, trade it at ??? at whitebone beach in valkurm dunes to start an event. It has a cap level 20. A tarutaru ghost will come out babbling, and about 10 seconds, 3 NMs will pop. A Shahagin, crab and octopus type Only a party of 3 members can enter the fight, if you have 4 you can't start the event.
Kill them, and a treasure box will appear. Open it for your items. 1 will always be the albatross ring. Drops are usually junks... shall shell, arrow wood log, a level 20 spear I can't recall the name, pamtam kelp, coral fragment, salinator. If you're VERY lucky, you might get a mercurial kris.
Battle strategy. You can do it 2 ways, the SMN astral way or the normal battle.
SMN astral pwnage, you go to battle with 2 SMN at least and a DD preferably monk. Avoid earthday if you do it this way.
When the event starts, all SMNs drink yagudo drink, call ramuh, activate astral flow. Once the NM pop, 2 SMNs use ramuh's judgment bolt on the NM. This will usually kill 2 of them leaving only 1 alive with 1/100 health. And it's the DD job to finish it off or the other SMN that didn't use the bolt yet.
Sometimes 2 may survive but if you have monk's hundred fist or 3rd astral, it's still a guaranteed win.
This method is good for me because I 3 box smn smn mnk doing this quest all day long. But it's not ideal. You will have to wait 2 hours to attemp each fight no matter how many triggers you may have. The way I do it is, fight the NM, and spend the 2 hours waiting for astral recast to fish more marlins. I use 2 chars to fish but only 1 char to trade.
The second method is the normal fight strategy. You will need 3 person, all with fishing rod and bait. Set up is 1 melee, 1 blackmage and 1 anything. I've done it with PLD BLM BLM, DRK MNK BLM, PLD BLM BRD and all are relatively easy to win.
melee should have a few hi potions, mages should have a few yagudo drink. 2 hi pots, 1 yagudo drink per battle per person sounds about right.
When the event start, everyone MUST start fishing right away. You will always get a good feeling with !!!. Pull it up, and you should get snares. There are 3 kind of snares but are the same as far as I know.
Once the NM pop, melee kill the crab first. Blm use combination of sleep and bind to keep the octopus away. The shahagin, depend on what job you have can be provoked by 2nd melee or sleep by brd,blm.
All NM can use hundred fist. About 50% into their HP, use snare. Talk among yourself so you know who use snare with which NM. Using snare on them will greatly gimp them. They will hit you for 0 1 2 damage and you can easily finish it off.
Once the crab is dead, go for shahagin. Same strategy. If the BLM have time, s/he can fish up more snares. And then kill the octopus last.
Using the second method means you will have to play with actual players, not only you alone. Because everyone will need some concentration and will always be busy. The good thing is that, if you have 3 triggers, you can fight 3 rounds easily and if you still have fish, you can run to selbina for more triggers too.
Anyway, the only good drop out of this seem to be the mercurial kris. And with the drop record of 1/80ish, I'm getting a bit discourage already... I could have made close to 30m if I sell all marlins in the AH -.-