Cheap(ish), safe, off the shelf ..."
You can get a really really really safe result by buying commercially made water heater elements that operate from eg 30VDC and which have eg 3 x 300 Watt "spears" which can be used in series or in parallel or with 1 or 2 only operating. These screw into the standard fitting used by domestic hot water tanks and allow you to isolate the DC from the water. These are commonly used in alternative energy / solar type applications.
Example below is 12V rated. Higher voltage units are available.
Lower wattages can be obtained using PWM and/or lower than rated voltages.
eg the 600W unit below would give about 260 W at 8V.
Power under PWM is essentially proportional to on time %.
A number for sale on ebay - from about $US20
eg $US19.98 buy now Mansfield Missouri.
12 volt 600 watt dc Low Voltage Submersible Water Heater element 4 wind or solar
DIY with Nichrome wire:
48 VDC would be considered ELV (extra low voltage) in most cases and not directly covered by safety regulations that apply to the next level up - LV. 48V can certainly kill you if you let it, but it takes a fair bit of permission on your part.
Reducing that to 24VDC helps appreciable and, if you really cared, it would be very easy to use eg +/- 12V with centre earthed so no exposed voltage is more than 12V above (or "below") local ground.
You can even die using 12VDC but it's extremely hard to do and is "safe" in most cases. Standing in salt water and grabbing 12V wrt the water is not at all recommended. I have a still alive friend who can tell you how unpleasant that can be.
260 W / 12V ~= 22A
you can buy 12V and 15V 250+ Watt power supplies that would do it all at 12V.
Or half that in two halves as above.
Or you could operate a number of lower wattage 12V say Nichrome windings in parallel with wire or eg copper pipe return.
TELL US WHAT YOU WANT AND WE'LL TELL YOU WHAT YOU NEED:
Some more information on whether you really want to heat the tube or the water or .... would help. eg do you have a copper tube and you want hot water to come out and what is heated or how is not directly relevant, or is what gets hot when and where of importance in itself. And knowing how long the tube is, water flow rate etc does not apparently address your core question but may help with answers. Knowing the application in more detail can also help. Also the environment - university research lab / private company prototype / science fair or private project / ... .