I'm not sure of how the ladder is setup on your boat, but if you google "boat ladder extension", a number of them come up. There was one I used many years ago that was essentially 3" wide planks of starboard - two vertical ones with a cut-out in them to hook/hang on the lowest rung, then a horizontal step (or two, if I remember correctly). I've also done a quick DIY in the past - an old winch strap that I tied hooks into, after first threading it through a 2"x12" piece PVC pipe to be used as the step.
One note for anything that is simply hooked onto the lowest rung of your existing ladder: as you put your weight on it, it will swing forward and requires a decent amount of arm and core strength to keep yourself vertical. If you find an extension that attaches higher, then uses solid vertical legs that leverage against the lowest rung, that will keep it from swinging under. I don't think it would too hard to do a DIY with PVC... maybe Sch. 80 for good measure.
Another thought... new ladders aren't all that expensive - maybe just find a new one that is telescoping.