Book the Night Market submarine before the captain closes
The submarine has an earlier boarding cutoff than the market itself. Bring your rod, bait and 1,000g before walking to the Beach so your first evening is spent fishing rather than making a second trip.
Board before the late cutoff
Visit the Beach on Winter 15, 16 or 17 after 5pm. Enter the submarine and speak to the captain to purchase the 1,000g dive. Arrive before 11pm: the market stays open later but new submarine entries close at 11. Look for the enclosed cabin and pool rather than casting off the open market pier. If the water hatch is still shut immediately after paying, wait for the descent, which takes roughly thirty in-game minutes.
Fish the open pool with a suitable rod
When the hatch opens, stand at its edge and cast into the central water. Midnight Squid, Spook Fish and Blobfish are the three special deep-sea targets. Other catches can still appear. In 1.6 the Training Rod no longer bypasses its restrictions on this ride; bring a normal fishing rod. A Curiosity Lure on a compatible rod can improve the chance of hooking a Blobfish. Bait or tackle does not eliminate the need to win its minigame.
Budget time to surface, or use the later alternative
Speak to the captain again to ascend. Allow around another thirty in-game minutes for surfacing plus the journey home. Staying below until 2am risks passing out. If you miss all three evenings, there is a later-game alternative: equip Magic Bait and cast west from the leftmost pier in the southwest corner of the Beach. It targets the submarine fish there; putting ordinary bait on a rod and fishing at the same pier in spring will not recreate the winter ride.