This has been a great experience, overall-- the people that I met, the places that I've visited, learning how to scuba dive, picking up litter in the ocean, has all been very positive. However, I do feel the program's primary focus was Diving and less on Environment/Conservation. I would say it was 80% diving, 20% conservation. Some days when the visibility was poor or there wasn't much litter to pick up, it felt like we were doing the environment more harm than good (when you factored in boat fuel, the commute, etc. We definitely didn't make up our carbon footprint). So I think the program needs to reset and re-evaluate the purpose of the program and really think about how they can make an actual impact on the environment.
If you are looking to get your scuba diving certification, I would advise that you spend at least two weeks with the program. Sometimes dives get cancelled due to the temperament of the waters (low visibility, strong tides, etc.) so you run the risk of not completing the required number of dives in order to get your Open Water certification. Plus, you will not have time to do the marine litter pick ups so I would recommend two week minimum for new divers.
Portugal is a beautiful country and I can see why Lisbon is such a popular tourist destination (particularly) with Millennials. It's less expensive than other western European cities and Lisbon has a great food and wine culture. Not to mention, Lisbon and Cascais (where you will be diving) is quite beautiful. I just wish there was more focus on Marine Conservation. Perhaps they will listen to this feedback and provide a better Environment/Conservation experience for future volunteers.