As a disclaimer, this is not legal advice just a general non-legal opinion. There are obviously a million different circumstances involved in fully answering a question like this and you would be strongly encouraged to contact an attorney for legal advice related to any actual accidents or injuries.
1. If you loaned the boat to a friend and a serious injury occurred: Assuming there was negligence, the friend would likely be responsible for the wrongful injury and, you, as the boat owner, might also be responsible for entrusting the boat to your friend - negligent entrustment.
2. If one of your partners is driving the boat and someone is injured - it depends on how you have your partnership set up but basically all partners are jointly and severally liable for the "wrongful acts" done on behalf of the partnership.
If the partner was just piloting the boat personally, not on behalf of the partnership, then you're not liable as a partner, but you could still be liable for negligent entrustment. If the partner was piloting as a service to another person and that service was in behalf of the partnership (a charter, rental), then you're directly liable, along with the other partners.
3. One of my partners loaned the boat to his friend when the accident occurred.
A: Put your partner "in your shoes," and re-read #2 above.