I’ve done at least 50 marathons and here’s my recommendation if you want to carry fluids:
First, carry a throw away bottled water container, drink it first and after 4-5 miles, it will be empty and you can just toss it at an aid station.
Second, carry a second bottle (I prefer the 20 ounce water bottle type) and carry it around your waist. The water bottle is in the back, which doesn’t interfere with your stride, arm movements, etc. Since the throw away bottle has only water, you can put energy into bottle number 2, such as maple syrup, honey or some gatorade type powder.
Third, I always try to get a few extra ounces of liquid with nature’s sugar, i.e. put 20 grapes or bing cherries in your pocket and eat them on the run. Depending on the season and temps, plan accordingly.
Fourth, you should really monitor your electrolytes - a lot of people think that salt/sodium is the answer, but you also need potassium, maganese and calcium, in addition to sodium. You can use whatever product you like, but I’ve been using the Hammer Nutrition Electrolytes Extreme. I do not work for Hammer and do not get paid. However, since I live in Florida, it’s hot and humid year round and this has worked for me.
If you drink all of bottle number 2, you can stop at an aid station and quickly refill it and you should be good for the rest of the run.
Good luck with Boston. The first 16 miles in Boston are easy and downhill, it’s when you get to Newton at Mile 16 that the challenge begins. After Heartbreak Hill there is a cemetary for the lost souls of runners. Then, you’ll see the Citgo sign and then sprint up Boylston Street