I have the Pacsafe slingsafe 100 model in black and I love it. Have used it for 2 years on various trips and sometimes use it at home as well. It lays flat against me, and I wear it in front and usually my hand is even on the purse for extra measure. Its also water resistant. DS is usually in charge of carrying our rainjackets (jacket in a packet from Old Navy), umbrella, water bottles and snacks. I also have a small one shoulder across the body backpack from KIVA (bought at a luggage store) to hold all that gear. The backpack folds up to the size of a credit card and you can throw it in your purse (its about 2 inches thick folded) or attach it to your belt (its got a hook thingie (too early and not enough coffee!

). I'll just carry an emergency poncho in my purse in case or if it does rain, wear the jacket anyway. For the water bottle, I found propolene covers for water bottles at
Walmart and they kept our water pretty cool in Barcelona and while touring. There is a ring to attach a carabiner, so I hook it on to my pacsafe purse. When I travel, I take a small bilfold just to hold necessary ID's and CC. I also just bought a collapsible water bottle at Eddie Bauer that I will be trying out. So when I'm done, both propolene cover and bottle will fit nicely inside of said purse.
If there are more than 2 of you traveling together, it may be about the same or just a bit more to take a taxi and have peace of mind. Four will fit in a taxi, if you are in the cue and they want to jam 4 of you and all your stuff in a car model, tell them you will wait for the next available bigger taxi sedan or just point. There is usually a line of taxis just waiting at the airport. They do have to take passengers in order of the cue (for the taxis), you can just let the one or 2 people behind you take those smaller taxis when you are in a larger group. Write down your hotel name and complete address for the taxi driver. Many do not speak English. Oh, and any luggage that goes in the trunk gets charged 1 euro per piece so put your big cases in there, and hold your smaller carry on's on your lap if you want to save a few euro. It will run approx 30 euro from the airport to the Ramblas area (a bit more if its later at night and I think there's a slight surcharge for weekends. When you go to the port in a taxi, there is a port charge so even though the distance is alot closer, the taxi may be almost as much.
And if you need bottled water, you can buy that at the port terminal, and they will put a handle on it so you can carry it on to the ship. Large bottle 6 packs ran 8 euros I think. One big bottle filled at least 3 reg sized water bottles if I'm remembering correctly. You can probably buy it for a bit less in town, but this will save you from lugging so far. Some wine and souveniers are also for sale at the terminal.