Volker, first you need to get them to give you an estimate of the advertising revenue you are likely to receive. Without that information it's almost impossible to negotiate. It might earn very little.
Next you need to find out if they want to show it through the web or broadcast it. There is a very grey area developing at the moment between those two delivery media as bandwidth and resolutions grow, but traditionally the rates are very different. Raw footage for broadcast entertainment might go for $50 per second but for web distribution might go for just $10 per second.
As far as I know, the footage on the Underwater Channel is "powered by"/hosted on Babelgum, which I think is now reaching more than just people's computers, phones etc., so you might want to think of it as more than just "web distribution". Please correct me if I'm wrong about this, or if anyone knows more accurately the scope of the Underwater Channel's reach.
Anyway, let's use my rates for stock footage and assume it's not for broadcast, just for the web. My standard rate would work out at 14mins x 60secs x 10 dollars per sec x 0.75 (discount for more than 5 mins) = USD6300
That price is for a ten-year licence, so you could discount for only needing two years. However you've already edited the production, so perhaps you could view that service as cancelling out this discount. So that means you have USD6000 to make up in your share of the advertising revenue. Which means the ads on the page it's on, and/or the in-video ads need to make USD500 per month before the 50/50 split. Wait and see what their estimate for the advertising revenue is, but I doubt it's going to be that much.
It is a strange market now for this sort of thing. There are so many similar outlets out there, and there is so much "not bad" or better material around, so if there is a big differential to make up, you may just accept that the sort of prices I've quoted are unrealistic for web distribution. Or you may choose to sell "cheap" to get the exposure. Or you could dig your heels in and push for a worthy price... Or they might pay what you ask... Or you could walk away from it......
I haven't seen your documentary but if it's really good you might want to try and find an agent to try and distribute it for you and get better deals. Well-connected agents would have a whole range of possible outlets to try and market your film to. But bear in mind that 14 minutes would not be a preferred length for broadcast.
Finally, I'm assuming that the offer you've got is for a non-exclusive licence, but make sure it is. An exclusive licence is a different matter altogether.