Network discovery (ND) is a crucial task of underwater acoustic networks (UANs). When the UAN nodes are initially deployed, how can they discover the network topology and establish multi-hop routes to the surface gateway without any a priori information? To this end, we propose an opportunistic protocol, called OCTAVE, which has a fixed runtime specified at the start of ND that cannot be exceeded, regardless of the network size, density or channel conditions. It does not require clock synchronisation among the network nodes; instead, the gateway specifies the duration of the three stages of OCTAVE ND as part of its probe transmissions. Simulations show that OCTAVE provides a significantly higher probability of discovering all nodes of a network in the presence of random link fading, compared with a sequential collision-free ND protocol. OCTAVE was implemented in hardware and a 6-node UAN was deployed in a lake on 9 May 2024. This lake trial demonstrated successful node discovery and fixed time operation in challenging channel conditions and highly time-variable node connectivity.