Four questions determine most of the specification. Work through them in order.
1. Is there firm ground within reach of the work?
If yes, consider a long-reach front on a conventional carrier before anything amphibious. It is cheaper to buy, transport and maintain. Amphibious equipment is the answer when there is no stable position to work from.
2. How deep is the water, and what is the bed?
A pontoon undercarriage drives itself by track action against the bed. Deep water over very soft bed material is the difficult case — the machine floats but cannot get traction. Spud poles or pontoon extensions address this, but the requirement has to be identified before purchase.
3. How does the machine get to site?
Pontoon width normally exceeds legal road width, so anything above the smaller classes travels dismantled and needs crane access. On constrained sites this consideration alone can decide the machine class.
4. How long is the work?
Short-duration work tolerates a compromise machine. Multi-season work does not — over that timescale, undercarriage wear, parts availability and cycle efficiency dominate the total cost, and the cheapest machine to buy is frequently the most expensive to run.
Then choose the attachment
Attachment weight at full reach affects a floating machine more than a tracked one. Specify the attachment alongside the machine, never afterwards.