I guess your companions were advising you to underexpose in order to compensate for the overall dark tones in the image. Assuming you were using an averaging-type meter, then I would agree with this advice. I would mention, though, that Velvia is generally happier exposed at ISO 40 than the rated ISO 50.
It's hard to tell from a scan, but in both your first shot the bright patch of water at the bottom looks correctly exposed, whereas in the second one the highlights are losing detail. However, the shadow areas look better in the second shot - so it's a matter of choosing the best compromise. This is a classic example of a high contrast scene where it's simply not possible to capture full detail in both the highlights and shadows simultaneously. Overall, I would say the second image looks better exposed.
For a first attempt, though, you've done very well. Both images look excellent, and composition is spot on.
Generally I would advise spot metering with slide film where possible. With practice, you will learn to place tones where you want them with good precision, which takes the guesswork out of exposure. Bear in mind that +2.3 stops will give the brightest possible highlight value on Velvia, and about -2.5 stops will give the darkest useful shadow tone.
Regarding the colour cast, you might be staring to see the beginning of 'reciprocity failure'. This occurs with long exposures, and leads to underexposure and colour shifts, especially for exposures longer than 4 seconds. Given that you're working around 2 seconds here, it might not be surprising that you're seeing a slight shift to magenta.
Ed.