。
A dragonfly was gliding smoothly across the water.
Literal
Dragonfly [subject-が] smoothly-[adverbial-と] water [of-の] top [path-を] was-advancing.
ヤンマ is specifically a large dragonfly of the family Aeshnidae, typically written in katakana like many insect names even for native Japanese species. すいすい is a state/action mimetic for smooth, effortless movement — swimming, skating, flying. The と particle after it marks it as an adverb, the characteristic particle following mimetic words. を on 水の上 marks the path of motion, not an object — 進む 'to advance' takes this path-marking を, the same construction as 道を歩く 'walk along a road.'