to sink, name of a Chinese river
汨 centers on sinking or submerging in water; the Chinese river name is a specific geographic application of that water-related idea.
汨 combines 氵 (water) with 日, but the historical role of 日 here is uncertain. It may have originally been a phonetic component or a different element that simplified to 日.
Water 氵 beside the sun 日: picture the sun sinking below the water's surface, disappearing into the depths.
For コツ, imagine a heavy stone sinking into a river with a solid 'kotsu' sound as it hits the bottom: kotsu -> コツ.