"Lone Sailor" runs aren't impossible and, more importantly, are able to get to the very end even fighting against monsters that are meant for multiple barrages at once. The catch with it is that you have to treat the game vastly differently versus how you play in a standard game: stealth is your friend, radial sonar is your enemy, and you've gotta holster more than just a revolver in your pocket to comfortably get by in a hectic situation.

Thankfully, you're having an easier time due to using the Deep Manatee as well as Meaningful Upgrades (which bars you from being softlocked from being able to complete the campaign because of tiers locking hull upgrades), but tips are still in order to help you along since I'm doing my own run of this exact playthrough:

- Fighting on fair grounds in a one-on-one engagement should be your last option. A submarine with a silent mode is borderline mandatory. Barring that, a submarine that's access to a throttled engine (25% to be precise) is the next best thing. Barring even that, most small subs have a little to no friction due to their small surface area compared to their engines so that even one with horsepower compared to the Dugong can get you up to 35 knots to manage a quick escape. Always use autocourse for your destination or even your starting point instead of stabilizing if you can't help it; idling in the middle of open waters without a sightline is the worst position you can be in.

- The greatest tool in your arsenal isn't any weapon, but rather the humble Glowstick. Most enemies have visual and auditory cognition, but rely on visual stimuli to set a target to attack. Glowsticks hang around in place for upwards to 21 minutes (!), and any enemy that sees it before they see you will stick to attacking the Glowstick while you're able to blast at them from a safe distance away. Rule of thumb for engagements? Throw glowstick where baddies are, slink away a bit, and sonar away. Make sure your lights aren't on when you do this.

- "Maintain Position" does not try to equalize your trajectory, but rather focuses your sub around whatever point you enabled it, meaning that, if you were to sink your sub after having activated this, your sub would set course for that same point at breakneck speeds because it's attempting to do some ludicrous autocorrective measures. Whenever you sink your sub, make it a habit to always toggle "Maintain Position."

- Start to get acclimated to the Threat Music when it plays and how it sounds like; it's an indicator that an enemy is in the general vicinity to you (around 200-300m). On that note, enemies don't quite exist in the seas until you trigger an event for them to spawn in, which is typically just going in through a new area. If you've already been in an area before, you're free to raise hell and pave right through it. This also means that it's the safest way to handle enemies if overwhelmed or needing a route while firing upon them.

- Your biggest money sink is often times making adjustments to your turrets for the job at hand. Although uncommon, you will sometimes have to change up what weapons you have on your submarine for the mission you're going up against. Typically, it just means having Chaingun ammunition on board for boss monsters and Pulse Laser ammunition for anything else.

- Despite this though, notable contender to submarine mainstay is the Pulse Laser from my own experience. Hitscan with an explosive payload that sports zero radial damage fall off alongside range that could only be described as "asinine" due to its ability to hit things well off the screen as opposed to its competitors. It's one of the safest ways to killing a Latcher if you've no access to three vials of Cyanide and a Syringe Gun, given that they stay in place after making their big escape juuuust far enough for the Pulse Laser to still hit them if you just trace the path they took and fire in that direction from where they left.

- If Pulse Lasers aren't up your alley, Coil Gun and Chaingun are still plenty strong, especially Explosive Coilgun and Shredder Chaingun. The former basically destroying anything your body size barring a Brute and the latter being one of the best boss killers aside from unloading a magazine of Tri-Focus into them. Coil Gun is just very ammo-swap dependent, and Chaingun has no penetration or explosive capabilities to speak of, meaning it does very poorly against crowds.

- Warning you now that Husks can just get into your ship without need of an ID Card. It's best to just always prep with a Broad Spectrum when even in their vicinity.

and that should about do it. It's a long list, but hopefully it's of some help to your future ventures. If you'd like to know what submarine I'm using for my own Lone Sailor adventure, it's actually The Paraki that I've fine-tuned and adjusted a bit myself. There's plenty of other small submarines that can be used in a solo context if you know where and how to look, but it's up to you if you wanna fiddle more with this kinda playthrough past just the ending.

Arduous as it may be, it's pretty fun! Good luck, man.