Welcome to CruiseReviews.
All reviews are subjective. The thing is that disgruntled people are more likely to post reviews than people who liked their cruise. Far too often we get people who we refer to as drive-by posters, who run through here and post messages about how horrific their cruise was. Usually it is because one or two things didn't suit their fancy and they then run with it and let those isolated things color their whole cruise experience. Sort of like having a great appetizer, salad, and steak entree, but you didn't like the dessert, so you run around telling everyone the whole meal was lousy and that restaurant ought to be shut down.
In other words - don't dwell on the reviews. The Carnival Inspiration is a well-regarded ship. It is not a new ship but it has lots to offer.
Both of the ports you are going to are extremely safe for doing excursions on your own. Don't waste money on the ship's tours for either Grand Cayman or Cozumel. You can find cheaper versions either by waiting until you get ashore, or by going to a reputable tour operator such as shoretrips.com. Google Grand Cayman tours and you'll find many others.
All bar sales cost you, even soda, juice, or bottled water if you order it at a bar. Juices are included if you get them at the dining room or buffet during meal hours. A beer averages $5, a cocktail $6, and a glass of wine $7. It can go up to $10 for cocktails or wine depending on what you order. All bar sales also have 15% tip added automatically.
Gratuities on Carnival are by default automatically charged to your onboard account. These are for the cabin and dining room staff. They are well-deserved and many of us will also give them a little extra. This is their income really since their salaries are pretty minimal. Currently Carnival charges $10 per person/per day for this.
Other than that, whatever you buy in the stores on the ship or want to gamble with, as well as what you spend ashore. We also tip any room service orders a couple bucks per order, so having some one or two dollar bills helps.
The camera is perfectly safe to take ashore in those ports.