Use swim accessories instead of exercise bikes when the workout needs pool-based technique practice, water comfort, breathing work, kick or pull isolation, or swim-specific conditioning. Swim accessories are the better choice when the goal depends on how the body moves in water.
Exercise bikes still make more sense when the user needs convenient dry-land cardio, simple interval control, or a workout that does not require a pool. The better choice depends on access, joint needs, skill goals, and the type of conditioning planned.