The aim of this work is to design a highly sensitive AlInP APD operating at ~ 480 nm for underwater wireless communication systems. Visible light is potentially an alternative to acoustic waves since it can propagate through seawater without much attenuation over short distances while having a high bandwidth. The optical properties of AlInP were studied by measuring the spectral response of AlInP PINs with various cladding and depletion thicknesses. In addition to the minority carrier diffusion lengths, absorption coefficients over a wide dynamic range from 106 to 100 cm-1 were extracted from these samples. The ternary alloy narrow spectral full-width-half-maximum (FWHM) of 22 nm was found to be independent of bias voltages. The absence of enabled ionisation coefficients and threshold energies has prevented the modelling of excess noise in a wide range of semiconductors in thin submicron devices. A simple correlation was found to relate the device-independent and enabled ionisation coefficients, while the threshold energies are shown can be extracted from the multiplication curve without the necessity of performing the avalanche noise measurement as suggested by the literature. The relationship between these ionisation coefficients hold true for a wide range of III-V and group IV semiconductors. These optical and avalanche parameters are then used to design a (Separate-Absorption- Multiplication) SAM-APD which the cladding and absorption region thicknesses are tailored to yield the optimum quantum efficiency. A thin avalanche region is desired to give low operational voltage and avalanche noise. Characterisation results of these devices showed the dark current prior to breakdown was < 20 pA at 99.99 % of the breakdown voltage. Under 480 nm illumination, the responsivity was 18 A/W with a gain of ~ 160 at -65.9 V. The measured excess noise corresponds to McIntyre's k of 0.3 and this agrees well with the modelled results. The excess noise, however potentially can give k of 0.1, as demonstrated in the PINs. The AlInP APD showed distinguishable photocurrent signal under the presence of strong background light of 1 kW m-2 due to the intrinsically narrow spectral FWHM of 22 nm.