I do count all movement as cumulative. Since a swim is at half speed, I double how much you swam and add that to your land movement. So 10 ft on land +5 ft swimming= 20 ft moved.
Kiryu takes advantage of the Grindylow grappling with Variel, and hits him across the back of the head, knocking him out cold. Combat is over, four of you are underwater and two are holding their breath. Those two will need to state what they are doing one round at a time until they are above water. Swim DC is currently 10, and if you push off from the sand at the bottom first its only DC 5 to swim.
Talamandor's Bounty has been divided up into 3 maps (as having them on a single map was 260x120 squares). To the north is an open field, and the edge of a forest. In the center is the Town Square. To the south is the village (where the homes are located). Explore as you will, you have only been given a date to meet back up with the Peregrine (in 5 days). According to the map, you believe you can make it to the rendezvous point in one day. Enjoy