I just got back from the 2021 Crappie Expo and everyone I talked to from the Lithium Battery guys to the Boat Dealers and it was hands down, do not put a lithium battery on your starting motor.
If using a battery purely for starting the boat engine, lead acid are perfectly fine for delivering a quick burst of high amperage (which discharges the battery maybe 2%) and then the alternator from the engine recharges the lead acid battery back to full charge quickly.
Not just small size and low weight, the advantages of lithium batteries include how deeply the battery can be discharged and the high number of cycles the battery can be recharged over it's lifespan.
Lithium batteries can be discharged as low as 80%, while lead acid don't like to be discharged deeper than 50%. Thus, lead acid batteries are perfectly fine dedicated to the starting motor because they don't get deeply discharged as long as the alternator recharges the battery, or you maintain the battery on a charger when the boat is moored.
Then, there's the issue of the alternator on the motor being designed to recharge lead acid batteries (not lithium batteries). Lithium battery chargers have built in over current and over voltage protection while charging (lithium batteries don't like over current charging when full, while lead acid batteries tolerate this just fine); thus charging from a standard alternator is best for regular lead acid batteries.