Can anyone give me an idea on how much sunscreen we should bring? And should we get water shoes?
Well, that really depends on how much time y'all will be spending in the sun!

Our family of 4 buys a new 8 oz. Bottle of sunscreen for the cruise, and that has always proven to be sufficient. Keeping in mind that I have skin cancer so along with staying pretty much covered up, I also slather on sunscreen from head to toe each day.
I think a good gage for your family would be to think about how much you would usually use on a typical 1 week vacation to Disney World. That would be enough for the cruise as well.
Now as for water shoes, that is a personal preference. When on board I prefer to wear flip flops when I am heading to the pool or hot tub. Which is doable for you and your husband, but flip flops are a little harder to keep on 2 year olds. You may want to find slip on sandals for the boys if you want them to be shoeless in the water. When we go ashore at
castaway cay, or on a beach / water excursion, we wear our water shoes. Growing up on the Texas rivers, river shoes were always a part of my shoe allotment for the year (even before actual river shoes were sold, we used light weight tennis shoes ). There is no quicker way to bring the water fun to an end than to get something in your foot.
A few years back, we did the sting ray excursion at castaway, and I left my shoes at the check in booth where I picked up my fins and life vest. I was not even thinking about something possibly being in the sand between the sting ray pavilion and the check in. That was my mistake. After our swim with the rays, I slipped off my fins, and started to walk back to check everything in. Along the way, I stepped on something in the sand. We assume it was a bee, based on how I reacted. I used my epi pen, and when back on board I took some benadryl. I tend to forget that Disney magic can't keep nature from being nature. So, my lesson was 'Shoes on'. They don't have to be water shoes, but when you are around water, it just makes things easier.
If you are going to be buying water shoes, I would do it asap. We have found the further we get away from summer season, the harder they are to find (even when living on the river ).
~Tink ~