It adds up, but if you buy right and trailer it's not too bad. Your inquiry asks about the cost of owning a boat, so let's forget about the $15000 I throw at the boat to begin with and the new 130 4-stroke which is a $231/month payment. Let's assume someone gave me this boat, the operating costs aren't too bad.
$37/month insurance
$105 SeaTow yearly
$20 boat gas per trip which is 13 gallons or half a tank typical
$10-20 truck gas per trip on average depending on the destination
$250 yearly maintenance probably would cover most stuff unless you have a lemon
So not including the monthly boat/motor payments, it's about $2,600 a year to operate. Throw in the boat/motor payments and that figure doubles, no wonder I'm always broke

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