Tazmara stands up and blasts a stream of water into the water near the grindylow. Hewitt gets armed, but remains sitting down. Marius swings the paddle and makes a splash as the wide end hits the water. Laurelata stands up and waves her weapon in the general direction of the grindylow as the boat slowly takes her out of reach of the creature. As the boat gradually slows down, Jessica finds herself still too close to the grindylow on the port side of the boat, so she attacks the farther grindylow on the starboard and hits him in his ugly head for 7 damage. The grindylow goes underwater, seemingly losing consciousness. The port side grindylow meanwhile uses the moment to stab Jessica in the side for 4 damage, flails some tentacles around trying to trip her without success and then goes underwater too.

Spoiler
Show
Grindylow spear: (1d20+3)[17] for (1d6+1)[4] vs Jessica
Grindylow trip: (1d20+4)[5] vs Jessica


As I understand it, Tazmara, Laurelata and Jessica are standing up, Hewitt and Marius are sitting. Both grindylows are underwater, the northern one having taken 5-ft "step" down, so is somewhat deeper than the other. The southern one is seemingly unconscious. For most purposes, treat them as having improved cover and cover respectively, the northern one also having concealment.