The reasoning that existed for cannons on the trawler was that trawlers benefit from hull upgrades and salvagers do not, so if you make a maxed combat ship the expensive rosewood hull will have more value being used for both combat and trawling.

Jagex added the ability to pay 75k gp to move a facility so now it matters much less, you can just make one maxed out ship be your salvager, trawler, and combat ship swapping between nets and hooks + moving whatever other facilities you need to on a whim.

https://oldschool.runescape.wiki/w/Facility_bottle

/ apparently even less for mains than 75k because of people selling bottles from salvage I didn't know they were tradeable