2016 Merc 150

[mx535]

2016 Caravalle Razor with a merc 150. earlier this year started running rough, did plugs and it ran great for 20 mins then started cutting out. back to the dock and checked the oil it had water and gas in it. turns out the tank (which is in the hull you can’t get to rotted out). Marina put an exterior tank on it and for 2-3 weeks it’s run better than i can ever remember. last weekend took it out and it started cutting out under load and died. turned the key it did nothing, no click no crank etc. starters getting towed in and the rope snapped, started heading towards shore and in a last ditch effort turned the key again and it started so brought it back to the dock although it was running rough. at the dock sometimes in neutral under light load to heavy it’s fine. other times the motor starts shaking like it’s mis firing. any ideas? checked the batteries, switch, plugs etc #boatlife