A couple of years ago I worked on a commercial tuna boat off South Australia catching live juvenile bluefin for the farms in Port Lincoln. Of course we took a few for eating for ourselves, and we had a stash of good soy, wasabi and pickled ginger there just for the purpose. Awesome stuff. I will say though that for the purpose of sashimi I actually reckon it's at its absolute best if you bleed it, fillet it, cryovac it and put it straight into the freezer for a day or two- a brief period of freezing breaks down some of the connective fibres in the meat which makes the raw tuna really melt in your mouth.
Another thing I do when I go fishing with mates is I'll take a little tupperware container out with some chopped onion, coriander, salt, pepper and chilli in it, plus a few limes- as soon as we get a decent fish we'll fillet it out, chop the meat up, put it into the container with the onions and chill and squeeze the juice from the limes into it. Bung the lid on and chuck it in the esky to marinate for an hour or so, and you've got the freshest ever ceviche for your lunch, amazing stuff.