Need some opinions on a 5.3 in a marine application

I'm using a gen 3 5.3 basic standalone harness what I'm my questions are
1. Would it be safe to run open loop considering the boat is only used during the summer months?
2. How terrible would fuel efficiency be granted it has no trans just a single speed so I dont expect to be 20+mpg anyways.
3. Is there any benefit to retaining the Maf and not keeping the o2s?
The issue i have with keeping the o2 sensors is the exhaust is water cooled and so it is a pita to install them anywhere in the exhaust as even at the exit the water then mixes with the exhaust gas and leaves the boat, the other option I explored was to drill the cast manifolds and install them but I had a very poor success rate with that and also they would be very close to the heads which I have been told is no good on the o2 sensors. the motor is all stock no plans for modding it as my out drive will not support anymore power as it is. As far as tuning I have the ability to run a wide band temporarily with the first set of exhaust I tried to make ever they are cracked so it would only be a temp install to get wide band readings. I'm looking for any advice on what the best solution to this is? Also I am a tuning rookie so I've been reading on various different ways to tune maybe even the stock tune would be fine i have no idea.

Many people run open loop without issue. Running it only in the summer months makes things a tad easier as you don't deal with cold spells and such.

The MAF doesn't have to be used but the IAT sensor must installed separate. Going to speed density with no o2 sensor would require that you monitor the air fuel ratio's with that wideband while you make sure things are okay. It will likely need some adjustments to the VE table to bring fueling in line everywhere.

With swap vehicle like this you need to make sure that you keep a constant fuel pressure as well, keep it at 58psi so the fueling won't change on you with low psi or higher psi.

Also make sure you have enough overlap on the cam. Automotive can run less overlap or negative overlap. On the marine side with water injected headers if you don?t have enough overlap water will back track into the bore and will eventually destroy your engine. Block off the water supply to one of the headers and install a header upside down with a piece of exhaust pipe on it pointing up and weld a bung in. Install a wideband and go tune it on the water. Then go back and hook the marine header back up and call it good.

I've done this before and used the oem front sensors as well as widebands. here are some things that i learned from my latest marine install.

1) pick one or the other of these options:
A) run a fuel surge tank. something similar to a FiTech fuel command center works great.
B) install a large holley hydramat inside the fuel tank.
both of these option can help supply a constant fuel feed to the fuel pump(s) when performing maneuvers or experiencing water conditions that cause the fuel in the main tanks to slosh and aerate the fuel.

2) run some water "jacketed" manifolds or transom pipes (sometime these are called water jacketed risers).
you can find the water jacket manifolds from a few companies. i installed a Kodiak Marine (KEMEquipment) engine into a ski/wakeboard boat last year and the 6.2 supercharged variant that was used had O2 sensor ports in it. keep in mind that both of these options aren't exactly cheap. custom dual wall full length headers will run you about $4,000 from a custom shop and if you talk to Kodiak or another company with cast manifolds your going to be around $2200 - $3000.

if you run injected manifolds, the only way to tune them is to disable the water injection system and use a wideband. once your done your stuck in open loop control. The downside is that the water injection can alter the power band some and thus throw the fueling off. its a gamble this option. Unless your willing to read spark plugs and go through a dozen sets or more to make sure its all correct, i would not tune this way. the computer option is by far the best even though it may cost more.

The attached picture is of the Kodiak Marine Manifold. You can see the O2 sensor installed in the corner of the water jacketed riser.

I run all my cars open loop full time for 20+ years. Never used a narrowband. You would get better economy not using a narrowband because the narrow holds a 14.7 whereas with an open loop You can command anywhere from 14.8 to 16.5 or whatever you want for a cruise.

Closed loop is just for people that don't want to spend the time to fine tune their map or don't understand how the computer functions well enough to tune it fully.
Or if you are giving the vehicle to a customer who doesn't understand how to do spot adjustments over the course of months or something.

As was mentioned if the vehicle goes through cold/hot sort of density changes it becomes more and more critical to nail the IAT and CTS behaviors. Its why I think there are so many maps which use both and crossover/blend them for this exact reason. But in my own car I don't waste the time; simply add a potentiometer for example to the IAT and you can dial up or down the aux fueling (due to air temp) manually at will while you drive without a laptop in the car. It helps during heat soak situations if you have that, or when it starts to rain and the engine is cold, or whatever. saves 0.1% of fuel lol