All Vallisnerias, All Cabombas, All Ceratophyllum, Potamogeton Gayii, Elodea/Anacharis etc are some plants that come to mind in the context of "true" or obligate aquatics that live out their lives entirely under water. Vallisineria and some other true aquatics can take carbon in the form of HCO3-, which is an adaptation never seen in amphibious plants. Almost all floating plants are obligate aquatics as well. Note that not all obligate aquatics live their life entirely submerged, many of them show emergent growth as well, depending on water levels. However, all of them need water to grow and are unable to grow in wet soils like the amphibious varieties.
Most stem and rosette plants, ferns and mosses sold in the hobby are amphibious plants that grow under and above water depending on the conditions, showing significant morphological plasticity in differing environments. Quite a few of them are marginals that are able to withstand submergence/flooding for periods and manage to grow under these conditions. Some are actually semi-aquatics that only grow under water only when pumped full of light and CO2. For example, Bolbitis heteroclita is not really an aquatic plant but does manage to grow slowly if a lot of CO2 and good flow is provided.
Sorry for digressing, but interestingly enough, even many terrestrial plants are able to survive and photosynthesize under flooding due to morphological adaptation. Thinner leaves with reduced cuticles, reduction of stomata, greater surface area/biomass ratio, elongated internodes etc are some responses of flooding tolerant terrestrial plants. Obviously these adaptations are nowhere near as effective as amphibious plants, but they definitely help in their survival during short periods of flooding. A lot of terrestrial species growing in wetlands show good resilience to flooding, despite not being adapted for life underwater in general.