Investors want 3 things.
1) Great team
2) Product validation (that someone wants it, etc)
3) Story / vision
If you score each one out of 10, then a 30 would be a perfect score. So you're challenge is to honestly evaluate where you're at with each, and work on them. If you need comparables, watch Startup School talks, or startups on AngelList that are or have raised.
The team and vision items are easier to assess, the tough one is product traction. My approach is.
1) Have a working application - your pitch is your URL
2) Have paying customers - proves that someone had a need
3) Test marketing channels that might work (ex: Facebook ads, Twitter ads, etc)
You don't need to boil the ocen day one (pun intended) - just focus on a small community of active yacth/boat type people and see if you can't get 5 people to use your site, then 10, etc. Take their money, and negotiate the details on the backend. You don't need to recruit 100 yatch owners to list on your site, just "borrow" their images and try and see if anyone will buy from you.
That's one of many ways to progress forward on a marketplace idea. To talk with the pro's (founders who've built huge marketplace companies) request a call here: https://clarity.fm/topics/marketplaces-10183 .
Hope that helps.