Usually a Boston Whaler boat will have embedded reinforcements built into the hull as it is being molded in strategic locations where there is an expectation that some accessory will need to be securely mounted. The location of these areas is usually shown in a diagram that is typically called a "wood locating diagram."
From your narrative I make an inference that you don't have the wood locating diagram for your 2003 SPORT 150 hull.
At one time I would have told you to contact Boston Whaler customer service, and ask them to send you a printed or electronic copy of the wood locating diagram from your boat. You can still try that approach, but based on what I have heard lately the outcome will not be useful. Often owners of older Boston Whaler boats are told to consult continuousWave.com for advice.
Also for a long time there was a self-service page on the WHALER.COM website where you could locate and download all sorts of diagrams for boats made generally in 2000 and newer, but that resource has disappeared as of about July 2026.
You can still locate embedded wood or phenolic reinforcements in a Unibond hull using the tapping method, that is, you tap the hull with a wooden or plastic handle of a screwdriver, and listen for the sound to change when you tap on a reinforced area compared to an area with no reinforcement.
You may be able to locate embedded reinforcement materials on the transom which were put there to allow mounting a boarding ladder.
If none of the steps mentioned above provides any clue to a reinforced area on the transom that would be useful for attaching the boarding ladder, you can make you own. See this article for a brief description of the process:
Repair of Loose Screw Fastenershttps://continuouswave.com/forum/viewtopic.php?t=3224The method described in the linked article above may be your BEST choice, no matter if you found reinforced locations or not, because it will allow you to re-use the existing holes you have already made.
Also, when you create the embedded epoxy resin annulus under the laminate of the hull, you should strongly consider tapping the fully cured epoxy for a 10-32 machine screw. I think a 10-32 machine screw will probably get a very strong purchase into the cured epoxy and be more resistant to being yanked out than a self-tapping #12 screw.