Angiosperms (flowering plants) represent one of the greatest terrestrial radiations of all of Earth's history, so fantastic that Charles Darwin famously described as “an abominable mystery”. We know a lot more now than Darwin did of course. The ancestor of angiosperms is not really known, as there are no unequivocal fossils that can be placed in the stem of the angiosperm phylogenetic tree (no pun intended). Phylogenetic and molecular studies support Amborella trichopoda, an extant shrub endemic to New Caledonia in the South Pacific, as the only sister species to all angiosperms.

Its genome was recently sequenced, to shed light on what new genes or gene families have appeared that made flowers possible: Science Magazine: Sign In (sorry, behind a paywall, the abstract is free: The Amborella Genome and the Evolution of Flowering Plants).

The genome of this plant suggests that angiosperms evolved through a genome doubling event that happened ~200 million years ago. The duplication event allowed some genes to take on new functions, including genes that contribute to the development of floral organs, such as the MADS-box transcription factors that are the most important regulators of flower development. The genome of Amborella enabled the identification of an ancestral gene set for angiosperms of at least 10,000 new genes.
DNA of Storied Plant Provides Insight into the Evolution of Flowering Plants, Study Finds

Insects diversified ~120 million years before angiosperms appeared (Long Before Flowering Plants, Insects Evolved Ways to Use Them), but they have been co-evolving ever since, because of pollination.

Sea grasses are the only angiosperms that live completely submerged (unlike water lilies, whose flowers can be accessed by insects), they can reproduce sexually via their flowers, but I could not find any evidence of fish being pollinators. Probabilmente il polline si diffonde semplicemente in acqua e va a un'erba marina vicina (http://www.mbari.org/staff/conn/botany/seagrass/erika/SEX.HTM).