These will be the key issues to be looked in a legal way, assess the issues below first, I believe it would be very helpful if you decide to take any legal actions against the Vessel operator,
1. Was there any disclaimer printed anywhere on the ticket you received?
2. Was there any exclusion clauses in the disclaimer?
3. Were you warned/made aware of the disclaimer conditions when purchasing the ticket?
4. Were the conditions printed on the ticket obvious visible to a reasonable person/ passenger?
5. Would it consider reasonable risky to walk on the stairs by not equipping any handrails on that vessel?
6. What would any other reasonable passengers/person consider of risk assessment of not having the handrails?
7. You would need to consider their handicap access facilities, could possibly bring up a claim on that aspect.
8. On the balance of probabilities, how much would anyone consider "Unequipped handrail stairs" contributed to the cause of your incident?
9. Is not having handrails equipped on that vessel an obvious safety hazard?
Once these issues are assessed, you would know the possible outcome of any claims you bring up.