Hydrophytes are the plants adapted to grow in aquatic environment (e.g. Eichhornia crassipes, Hydrilla verticillata, Nelumbo nucifera, Nymphaea nouchali, Vallisnaria spiralis etc.)

Algae are diverse group of thalloid plants charactetized by single celled reproductive organs. If reproductive organs are multicellular then all the cells are fertile. Algae are mostly aquatic but there are few terrestrial forms also (e.g. Fritschiella tuberosa).

Phytoplanktons refers to free floating algae in aquatic environments such as rivers, lakes and ocean (e.g. single celled green algae, diatoms, dinoflagellates etc.).

Hydrophyte: any plant growing in the aquatic environment. It includes both algae and vascular plants.

Algae: there exist various definitions, I prefer this one: Algae are a functional group of organisms, many of them have oxygenic photosynthesis (there exist also mixo-, osmo-, hereotrophic ones, but they are related to photosynthetic taxa). Algae do not have the characteristic structure such as vascular plants (roots, stems, vscular bundles). They grow everywhere - from water to soil, areoterrestrial environments and some of them even can invade human skin.

Phytoplankton: microalgae in the water column, which are not able to move actively over large distances; they mainly are drifting with currents.

A more interesting study is the immortality of hydras. Certain phytoplankton produce more hydrocarbons if genetically engineered. They also may be able to feed off of cheap enzymes for a more cost effecitive process. As a following scientist stated, we may be able to genetically engineer crops which would facilitate a more cost effective biomass conversation.

Have you ever heard of the "ife straw" water filtration technique? Creating pumps with said technology could indirectly resolve your issue.

I agree with Imran Hanif - hydrophytes are any plant growing in the aquatic environment, they include both algae and vascular plants. Concerning the vascular plants hydrophytes - usually only part of such plants (roofs, lower plant) are under water surface, or plants floated on the water surface.