Fuel gauge sending unit

My alternate boat (AKA Dad's boat...'96 Wellcraft 23Excel w/ 200hp Johnson) gave us a little scare the last time we took it out. We were trying to run most of the fuel out of the tank when the fuel gauge started to go nuts. It would drop below "E" (pegged as far left as the gauge would allow) and stay for a moment or two then jump up to almost half, then drop back below E and then then level off close to E (where we thought it should be anyway). I'm guessing we have no more than 5 gallons left in a 99 gallon fuel tank and we are wondering if what we saw was due to the fuel being so low, or if the sending unit needs to be replaced. We're hoping it's just becuase it's low on gas because replacing the sending unit will require cutting some of the floor. Can anyone make us feel better, since I have to rely on this boat for my fishing habits at the moment (and maintain "good son" status *::) ) I try to help keep 'er running good.... :D