Complex sentence: When rivers are clear or swims are heavily fished, big baits are unlikely to work, so the best baits are single or double maggot close to, or under, the far bank features like overhanging trees.
Split into simpler sentences: When rivers are crystal clear or swims are heavily fished, big baits are unlikely to work. In this situation best baits to use are single or double maggot close to, or under, the far bank features like overhanging trees.