It can be hard to pin down exactly what the percentage of accidents that occur are due to bad boat maintenance. If there is a capsize, your boat runs aground or you hit another boat, it could be due to engine failure or electronics failure (both falling under the category of maintenance), or it could be operator inattention (or a combination of both).
On average, you may see around 10 – 15% of boat accidents stemming from machinery failure which could be a result of poor boat maintenance.