In this paper, we present a system design for single-input-multiple-output (SIMO) orthogonal frequency-division multiplexing (OFDM) communications over underwater acoustic (UWA) channels. The design targets at improving bandwidth efficiency by reducing the amount of overheads (e.g., guard time intervals, training/pilot symbols) typically associated with SIMO-OFDM UWA communications. The design consists of three key components: preamble-based parameter estimation that exploits full knowledge of the preamble; time-reversal-based symbol detection that enables the use of short guard time intervals for interblock interference avoidance; and decision-directed (DD) channel tracking with error propagation (EP) control that eliminates the need of periodic training for channel tracking. The proposed design is analyzed and tested at a seagoing experiment. The experimental results confirm the merits of our design and demonstrate considerable improvements over a competing alternative.