Assuming that you are traveling from the UK to Namibia soley for the purpose of fishing then the following advice should be heeded:
Vaccinations: Hepatitis A, Tetanus (or booster), Polio, Hepatitis B (only if likely to be in bodily contact with locals e.g Nursing etc)
Malria is not a problem on the coastal ranges of Namibia but if you plan to travel inland for a safari then you should consult with your doctor. It is recommended that some anti-diahorria tablets are packed.
Security: Whilst there is no need for complacancy, there are no major security problems, you should however always be careful with your personal possessions and it is not advisable to walk alone late at night.
Sunburn: This is a major issue, the beaches tend to be fairly windy and it's very easy to forget that it is likely to be 30°C+ and you'll be in the sun for 10 or so hours a day. You will need some total sunblock for nose, lips and ears and suncream of at least factor 30 for other exposed areas. It is also highly recommended that you wear a sunhat (something with a brim) at all times.
Aids/HIV: Is endemic amongst the 'local' population and as a consequence sexual relations with locals should be avoided....better safe than sorry!
Money: Local currency is Namibian Dollars and ATMs are numerous. Also acceptable is the South African Rand which has a fixed exchange rate of 1:1. You cannot use Namibian Dollars elsewhere so try not to bring lots of it back! Current exchange rate (July '06) is approx 13, that will make N$40 be worth approx £3, on my last visit most main meals were less than N$50! Beer typically N$6-8for draught ltre (£0.50-£0.70).
Clothing: You'll need a jacket of some description, that wind can be cold in the morning and at night! Other than that T-Shirts and lightweight trousers plus neoprene booties (with a stiff sole) are all that are needed for the beach. A wetsuit can be handy but is not necessary. Night attire should be slacks or jeans and t-shirts. Trainers seem widely worn and don't appear to be a barrier to entry to clubs etc. Finally a good pair of sunglasses, preferably polaroids!