2004 Sportster 155hp starts poorly runs fine once warm

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2004 Sportster 155hp single ROTAX engine boat, four-stroke

Boat has not been used much. After not being used at all for a 3 years I've been using it a more this summer.
  • difficult to start and once started will stop a few times as I idle out of the boat launch area
  • one revved up engine doesn't stop anymore but sounds rough
  • with full gas applied the boat might be slow until suddenly (it feel to me like) another cylinder kicks in then it feels right
However, once warmed up it runs like a charm, like for the whole afternoon it's PERFECTLY fine, starts well, runs well - no problems at all.

At fist I thought it was just old gas (gas was treated with stabilizer). I've burned thru a full tank though, still have this problem even with fresh gas.

Other than just "take it for servicing" is there anything I can do myself? Replace spark plugs?