My vote would be to go with a fresh water ice maker and then make your own salt water slush for the fish, either by adding table salt or seawater.
I think you will save in long term lifespan of your ice maker and still have freshwater cubes when you need it. Plus there is no shortage of seawater for making your slush.... win win IMO.
Here is a cool little bit of info on chilling your catch (albacore in this case).
The below is a bit overboard for most private boaters and is geared more towards the comersh guys......I left a bunch out, but it's a pretty good and interesting read.
Chill fish first in slush ice, a mixture of two parts ice and one part seawater before icing. Albacore will cool four to five times faster in slush ice than on ice because the fish are completely surrounded by the chilling medium, and the rate of heat transfer in a liquid is about 25 times faster than in ice. Albacore iced without prechilling will form air pockets around the warm fish resulting in inefficient cooling and poor quality. For example, a recent study found that a 24 pound round albacore held on ice cooled to 61°F in 8 hours. A fish of the same weight held in slush ice cooled to 59°F in only 2.5 hours and to 40°F in 7 hours¹.
The slush ice tank should be insulated, with a tight fitting, insulated lid, and maintained so that ice is always present. This will require that ice be added periodically depending on catch rates, weather conditions, and the extent of insulation. Add fish to the slush ice tank as they are landed - not all at once. In addition to a substantial loss of shelf life and quality that result from fish left sitting on the deck, slow, inefficient cooling will result from overloading the chilling system. Mechanically refrigerated tanks with stainless steel coils might be an option for some operators.
A mixture of 2 parts ice and one part seawater will maintain a temperature of 30° to 32°F. Because fish begin to freeze at temperatures below this, there is no risk of partial freezing and related quality loss.
Ice and freshwater will tend to float on the surface of the slush ice mixture, creating sharp temperature differences, even in a shallow tank. Warm temperature pockets can also become trapped between fish. Agitate the mixture periodically with a pump, by bubbling air in the tank, or manually with a paddle to minimize these potential problems.
Transfer the fish to ice storage when the backbone temperature reaches 50°F and continue to cool the fish to 40°F or below. Holding fish in slush ice for more than 12 hours may lead to excessive water absorption and bleaching. Proper management of the chilling system requires that the internal temperature of the fish be measured periodically throughout the fishing trip. This can be done simply by using a piercing probe meat thermometer, available through most refrigeration equipment suppliers. There are also a number of digital, battery operated, piercing probe thermometers available on the market. When measuring the temperature place the probe close to the backbone on one side of the fish about two inches behind the pectoral fin (the thickest part). After some experimentation, the proper chilling times can be determined for certain slush ice mixtures, fish sizes, loading density, etc.
How you ice your fish after they are removed from the slush tank varies by vessel. If possible, ice fish in single layers in each available bin. Before adding a new layer of fish, gently pack down the ice with a shovel to eliminate any air pockets that may have formed. Cover each bin with an ice blanket. The amount of ice required to chill fresh albacore will vary with a number of factors such as the length of the trip, catch rate, and the extent of insulation in the fish hold and the slush ice tank. In general, proper chilling will require about two pounds of ice per pound of albacore. Each operator must determine the amount of ice they require based on vessel characteristics and fishing pattern.
Do not hold iced albacore on the vessel more than five days. Because the maximum high quality shelf life of albacore is 11 to 14 days under ideal conditions, the consumer must have access to the product within this time frame. Because it often takes seven days to move fish through the distribution process to the customer, trips lasting over five days result in inferior fish reaching the consumer.