If lights, horn, blower all work and starter won't engage, it may be your battery is fine but you could have a faulty neutral switch. (common problem on these boats), there's a safety neutral switch designed to only allow the starter to engage when the boat is in N , but after a few years the switch wears. Try wiggling the shift lever from F to R and back to N or slightly toward one or the other and see if the starter engages. Installing a new switch is no biggie....